What is it?

What is it that makes some people so sure of themselves that nothing, NOTHING, seems to ever get them down, and some people (most actually) are so easily distracted by the things that COULD get them down that they lose all but a fraction of the hope they had once upon a time? This is the biggest puzzle to me, and though I really shouldn't be so affected by it I think of it almost continously. I'm sad about what people give in to, especially if I know and love and care for them. So what is it?

It's not circumstance, though everyone is quick to blame circumstances. That's a poor excuse because everyone's life is balanced with ups and downs, and everyone is affected by that bumpy rollercoaster equally. (Different trial, perhaps, but we are all given enough blessings to out-way those trials whether we choose to acknoledge them or not.) It can't be wealth either, because - as powerful of a tool as it can be - happiness is a choice, and comes without a price. So what is it?