Well I imagine it's a combination of things. Largely because he's extremely popular right now and has the opportunity to get a wide variety of roles. The longer he stays the greater the risk of being typecast. Plus he's been having back problems and the role is very physically strenuous. I'm bummed he's leaving but also very curious to meet another incarnation of the Doctor.

I agree with what harpyr said

i think its because he will wanna move onto bigger things. altough i dunno wat is bigger than dr who

Well, like harpyr said - he can get almost any role now that he's the 10th doctor.
But I think it's also the fact that he wants to move on... Doctor Who has been one great adventure for him and he will be forever remembered as The Doctor, but imagine you've been working on something for over 4 years... It must be so tiring for him.
And he deserves to settle down too... But I can't imagine David as a father...:D

I think it's because Russel T Davies and Julie Gardner are leaving. I don't think he'd be unhappy being the doctor with Stephen Moffat in charge, but I think he probably felt with them leaving, it was a good time to leave too.
I've heard him say in a few interviews that he should leave now that his character is so beloved and on a high. That he would stay forever, but thinks people will want him to leave after a while.
I COMPLETELY disagree! I think i could deal with him being the doctor for many years to come!