I disagree with the accepted answer.

> It seems to me that it's 'expected' that you give some sort of reason / feedback for leaving.

No. In my opinion and experience, leaving especially is a very, very formalized process because that is the *one* thing your employer has no control over, at all. You hand in your notice with *no reason whatsoever*, as matter-of-fact as possible, and in the unlikely case that they try hard, you say the magic word "no". Be sure to check if there is *any* response for the unlikely case that the letter got lost on the way. Obviously, you can inform them on multiple channels (e.g., a formal letter to HR, and a separate email to your boss "I handed in my resignation today, please let me know what you want me to do in my last X weeks"). It is the letter to HR that is the important one, and it is important that you get a formal response from HR.

The standard reason is "for private reasons", which is both the truth and pretty much unattackable. If they start to nag, then you can say things like "I have been offered a pretty exciting opportunity" (only if you actually have another job lined up!) or "I decided to change some things in my life" (if you have no other job lined up, so you do not have to lie about anything). As uncommitted as possible, not lieing, without giving *anything* away, and making clear that you do not wish to talk about it.

If your manager still keeps at it, like "is it related to topics X, Y and Z you asked about" then you can say something like "sure, if those had been resolved in time, it would have been harder for me to decide to leave", both truthfully acknowledging it, and making sure that it's too late now. If he then goes on like nothing had happened and tries to talk about X, Y and Z with you, quickly end the topic "I am not too worried about X, Y and Z for my last few weeks - how do you want me to hand off my work, and to whom"?

At no point should you (feel the need to) lie, nor should you talk more about *why* you are leaving. There is simply no reason to do so, the decision already has been made. If they are in the least professional, they know that and will know that it is futile.