Don't read on until after watching the movie. ;)
We often try to do something we think would be awesome. Until someone else comes along and tells us, "you can't do that." Sometimes it is a good thing, such as when our parents are teaching us not to steal candy bars from the grocery store.
However as we grow older and our ideas are a little more logical, we still get told that we either can't or shouldn't do certain things. We then start to place limits on our own abilities. (yes abilities is intended to be plural.)
Brother Mendez in the movie was always able to see the Magnificent Potential in other people. Sometimes it is hard, as in Will's case, to see our own potential, but as we strive to recognize the potential in others, we will find that we too have that potential within ourselves.
It is our job to improve our situations, occasionally someone else will reach out with a helping hand, but most of the time we still have to do something about it.
The great part is that you are always completely capable of doing what seems impossible because . . .