Personally I think environment plays a huge roll in this as well. If you grow up in an environment that constantly enforces that learning/growing/etc. are pointless, most people will eventually stop trying. Your environment also has a lot to do with forming your "id" [1]. I personally believe that if you could go back in time, take this same person and put them in a different environment they would be recognizable only in appearance.

I think most people are capable of being so much more than they are.

[1] pg has an article that talks about "id". Your environment will often influence how much you allow to become part of your "id" and what to do when various parts of your "id" are impugned.