So I'm a Junior Developer, I feel as though I'm a very happy person and I feel like I'm smiling. I love my job and the people I work with; I'm not unhappy. However, my new manager keeps telling me that I look unhappy and miserable and asks me to smile more.
Is there any way I can respond to my manager when he mentions this without causing offense?
I have what's I like to call "resting jerk face". I've heard women call it "resting bitch face". I've had your exact situation before.
What I do is explain to them "resting jerk face" is just my natural relaxed face and then I assure them that I'm happy and that I can't really change how I look. I then say that I don't want to fake a smile because I don't fake well. I also say that what looks irritated to other people is just me thinking really hard.
The reality is that they will get used to it. Just be consistent and say that you can't help it and reassure them that you're a happy person.
In my personal experience, you're not likely going to lose your job because your boss thinks you don't smile enough as a programmer, though as some commenters have stated, that's not always certain:
Word of caution on that last point, when my last company had to lay off people throughout multiple departments, the manager of my team picked the 2 people who seemed the most negative. One really was a downer, the other though was more like you describe. Unfortunately he was a pretty closed person, so the manager only saw the "resting jerk face", and we lost the most experienced guy on a particular product rebuild.
and
I was almost put on a PIP because I didn't like to high five and chest bump co-workers (although never left them "hanging") the manager at a time told me that "perception is reality". Obviously I shortly left that company and found a much better one.
You have a few options (UK specific edition)
Option 3 is not the best one, tbh. Use it only if options 1 and 2 do not work (try option 1 first)
Since you wanted some ideas on possible responses:
What makes you say that? [wait for input]
Oh I'm sorry, was I frowning? [wait for input]
How odd, I certainly don't feel miserable.
That's strange, I feel [fine / great] today.
Oh I was just lost in thought, now about that X project for William...
Oh I don't always externalise my emotions, but that certainly doesn't mean that I'm [unhappy / sad].
The core of any response here is to keep it light and to not make too big of a deal out of these comments (at first anyway). I'd go for one of the first responses since it could help you find out why your manager is getting this impression but the other phrases are less confrontational. I'd only use the last response if you've had similar comments before.
Regardless, I'd follow most of these up with some variation on the following:
Rest assured that I'll talk to you if I'm [having trouble with a project here / out of my depth / struggling with X].
Keep this about work topics because you also don't want your message to be "I'll come to you if I'm feeling sad / depressed".
Responding in this fashion should hopefully get these comments to stop in short order, but if they don't you probably need to address the bigger picture because it's very strange for your manager to think this, let alone to keep bringing it up after you've assured him that you're not secretly miserable.
Hey, you've commented a few times that I look unhappy and should smile more but I'm not sure what to make of that. Like I've said, I am happy to be here, I enjoy the work I do and get along great with my colleagues. I really don't know what signals you're seeing that I'm secretly unhappy. Can you tell me [what's giving you the impression that I'm not happy here / what it is that you keep noticing]?
It might be a weird conversation to have but at that point it's something you need to hash out with your manager.
There are a couple issues here:
If your manager were just concerned about your happiness, he would try to address that (at least with an inane and totally unhelpful "Cheer up!") so you have to address #3. The next time he brings this up, reassure him that you are happy and ask him why he thinks it's important for you to be (more) smiley. If it's for a stupid and possibly sexist reason, as some have suggested, then probably one of two things will happen. Either he'll be too embarrassed to admit it, and probably drop the topic, or he'll just present his stupid/sexist reason openly. The second gives you the opportunity to say, "I don't think that's reasonable; I was hired to do develop software, not to [look pretty/make everyone else feel nice/whatever his stupid reason is]."
If he insists that he's just concerned about your happiness, reassure him again that you are happy, and feel free to ignore his future weird demands that you smile. (If this will impact the rest of your career, you probably won't want to stay there anyway.)
Timing could not be better
Monday is Halloween - wear a smiley mask
Then leave it on your desk and put it on when he asks you to smile 
This expresses how how silly the request is, and I think it would just be fun. It is something I would do for the right boss. But beware that if your boss takes it wrong it could back fire and cause you more problems.
Next time your manager says something like this to you ask them if you can meet with them one on one privately. Explain that you are concerned that he has made this comment several times and that you are afraid that you are failing to meet some expectation. Is there some reason that he is asking you to smile? Are other people commenting on it? If your appearance is creating a distraction is there something we can do to minimize the problem since you are not unhappy, and are not intending to convey that you are?
Doing this will constructively address the root of the problem. If your manager is just concerned that one of his better performers is just not happy then you can allay those concerns and hopefully improve your relationship with your manager. If there is some problem festering you can work with your manager to find a solution that will improve the situation, that is far more effective than you smiling when prompted. And if it is just your manager clumsily attempting to engage you with banter and chit chat this should open up a dialog that will hopefully make things less uncomfortable.
If all of that fails and you still feel uncomfortable then it may be time to involve HR. I would approach them not as an attempt to get anyone in trouble but rather as an attempt to improve your working relationship with your manager. If HR can not help smooth out these issues they may be able to find you another position on a different team that might be a better fit for you.
And after all this you could still find that the situation is just as or even more uncomfortable. Then it will be time for you to decide if you want to just try to endure and hope it gets better, or find a new position where you fit in better, and your manager does not make you uncomfortable.
Your boss may be simply concerned that you are unsatisfied with your job.
The comment that you should smile more may simply be an attempt to allow you to loosen up/relax/enjoy things more--rather than a literal request to change your facial expression.
As someone who has received similar suggestions in the past, that is how I took it in my situation. It was intended for my own well-being, rather than an external requirement. (It's not so helpful in that regard, admittedly, but many people don't quite know how to interact with someone who is quite/reserved/etc.).
The boss may be worried, both out of personal concern for you, and because keeping employees happy is key to their own success.
If this is the case, try to reassure your boss that you are happy at work. I would say something like:
Don't worry, I really am happy here in this job. I just might not show it the same way other people do. I appreciate how I have the opportunity to do XYZ, and like the atmosphere here, etc.
Adding some specifics would be helpful, because a general comment "I am happy" to your boss isn't really that convincing.
I know that your boss is a man but I don't know your gender. I don't see an answer where anyone here says that this could be his way of flirting with you. If you are female, I would definitely consider this as a possible source of the constant remarks. I've seen it before where a guy will tell a girl to smile more as a way of flirting with her (over and over again). It's sort of like lazy flirting without trying too hard to actually flirt and possibly get in trouble (boss/employee relationship).
If you're a male, it still could be flirting - just with a different dynamic.
As to what to do about it? If it turns out he's trying to flirt with you, it is a whole different issue than if he is just making comments about your happiness. A lot of the other answers on here will tell you what you might do about the comments, but what if it is flirting?
If it is flirting... I would act really uncomfortable and ignore him when he makes those comments. If he doesn't get the picture then I would talk to HR if it bothered me that much.
Speak to your colleagues and find out if they agree that you don't look happy. Sometimes people can smile but the rest of their body language is saying something else. If you discover that you do really have a problem with expressing your happiness, consider acting classes or a good book on NLP.
I used to get the same thing a lot when I was high school - I don't know exactly why, but people always thought I was upset about something and said I should smile more.
The response I learned to give was simply to smile and nod. This may sound flippant, but I'm entirely serious. Don't make a big issue of it, just smile and move on.
The point is: there's not much you can do about other people's opinions of you. I mean, you can try to accommodate their feedback or "pretend" to be something you're not, but I find that sort of behaviour exhausting, personally. I'd rather just be myself and let other people deal with it.
If it were me, I would make an effort to make my smile as creepy and unnerving as possible in the hopes that the manager would say "never mind. belay that order." In any event, I would promptly ignore it.
There could be a cultural element you could claim though. In many places when a man smiles it is considered a sign that he is lying or intending to do you harm. Just say something like "i'm sorry, people from my cultural background do not express happiness that way."
