I think most of these answers cover it for the bigger part of "On screen" reasons, but I think it might be more than that. If you look at the progression of doctors, they seemingly get "younger" each time they regenerate. Imagine a big brother/little brother situation where everyone you know and love sees you as living up to the person you were the previous time. Not only would you have a HUGE role to fill, but with a new look, everyone would be expecting to see new bigger better things. The Doctor (at least recently) has had a very flamboyant take on everything he has done, from running away :P to serious moments like the death of a friend. If he thought he had to outdo himself with every new incarnation, it is probably the most stressful times of his life (even moreso than fighting off the Daleks and the Master). He has always been good at figuring out tangible problems, but has never really been one for facing emotional problems. He just runs away.

Onto another point I wanted to make. Even though every incarnation has been relatively the same, we have no reason to believe he will always regenerate into a "humaneske" figure like we have seen. I know the Doctor's rule (the Doctor Lies), but think about it... every time he regenerates, he checks everything out "I have a nose! I've had worse." (Tenth to Eleventh). the Ninth doctor made reference to "Having two heads, or no heads" and some other stuff hinting that he could look like other species when he regenerates. I don't know if there has been more on it in the series or not, but from my knowledge it could happen.

Last thing. Each incarnation, like stated above, has it's own personality. Maybe the Doctor is scared of losing what each experience has made him. Just having the knowledge of something that transpires teaches nothing. Every history class I have ever been in has taught me that. I think he isn't sure that he will still be "good" or even that he will want things like companionship and love. The thought of being alone scares the Doctor, and not knowing he's alone or not caring I think scares him even more.

Hope these help!