That brings up an old memory for me: I used to work as a lifeguard at a pool for three or four years as I was going through uni. Nothing glamorous in that work, it's just very loosely managing young males showing off in front of young females. Except this one time some kid jumped in the deep end and whilst he surfaced, he couldn't reach the top of the side of the pool in order to stay at the surface, he was clutching at the side with that unmistakeable, wide-eyed panic face. I reached down, pulled him out, and he ran off to his mum / friends / whatever, I never saw him again. Was a three second interaction from seeing it to him being back on dry land.
Possibly the best thing I've ever done in my life, and it was in my late teens. I get satisfaction from my intellectuality and general smarts, but for it really brighten my soul, it has to positively affect other people.
The bottom line is a demanding bitch and no matter how much you give it will ask for ever more. A positive change in a fellow human being is intrinsically, life-affirmingly satisfying; even just a single-serve in a lifetime.