In both cases you should talk about the things that attract you to the new company, but you could start with:
And what's to stop you from leaving here in lets say 3 years after
you've decided you aren't challenged here anymore? Not everyday can be
"I think your company offers excellent opportunities for career development over much more than 3 years. My previous company isn't able to offer me the step that this job represents for me. In 3 years I will be looking to take on more responsibility within your company".
Looks like you've done a lot where you are, how can you be bored?
"I wouldn't say that I'm bored at work, but this job interests me more because of the challenges it provides. I've mastered my old role, and achieved a lot in doing so. However, it's important to me to develop new skills, not just repeat old ones"
Actually I think this question is kind of silly if taken literally. Obviously you can be bored after doing a lot, even if you weren't bored while doing it. However, it may be that they think they've caught you out in something: if your CV says that you've spent the last two months doing something amazingly exciting and challenging from which you learned XYZ, then to say that you've run out of challenges immediately afterwards might look like you're either judging your old employer too quickly or else making it up as you go along.
I agree with others that it's probably intended to provoke you, rather than as a genuine request for an answer to an almost-philosophical question about the nature of boredom. They want you to make clear why they're a better prospect, because they want to hire someone who wants to join their company.
Assuming it's true, if there's anything wrong with your first answer, it's that it's kind of clichéed. Anyone can say that the position they're applying for is their dream job right now, and that they're always looking for challenges. Be more specific. Also make sure that you don't give the impression you think that doing something well is beneath you, and you're solely looking to do things you aren't qualified for ;-)
Probably another cliché, but one "right" answer to the interview question "where do you see yourself in 5 years time?" is, "in your job". Assuming the interviewer is themselves planning to move up, they want employees who want to move up too. If an employer declines to hire you because you want new challenges, then it's because they believe the job you're interviewing for is unchallenging. They're probably right. Don't say you want new challenges if you're applying for a dead-end manual labour job with no prospects. Luckily for you, most employers don't think that the jobs they're offering are dead ends. If anything they err in the opposite direction.
From a good employer's POV, the only thing worse than an employee who wants a better job in 3-5 years is an employee who doesn't want a better job in 3-5 years. It's just about how you present it.