Well, the thing is, I believe this song has so much more depth than people give it credit for. Sadly, a lot of it was lost when they tacked it onto a movie, but the actual message in it is simple, but fairly deep.
It's essentially about a man (or woman, to be fair) who feels overwhelmed, like even heaven itself were oblivious to their plight. But they know that they have a responsibility to do something to help those around them, even if they feel they can't ever do enough. And in the end, they feel that it's still not quite enough, that they've given all they can, but it may not be sufficient, that they may not have earned the love or respect of the people they helped because of it. But they keep going.
For no other reason than they know they have to.