I haven't a frggin' clue. I don't feel especially compelled to hand a homeless person a $20 when they're asking for chump change in the name of what's right (alturism), or to donate money to a family whose baby girl needs surgery after getting in some sort of freak accident, I just do.
No 'warm fuzzy' feelings, no 'I made a difference todays,' hell, I actually get mad at people that try to make a big deal about it and try to go on about how I'm a 'saint' for my generosity when it comes to helping "friends" (I use that term subjectively, as I don't actually consider anyone I know to be my friend, certainly a result of my light nihilism) and strangers, so there's no ego-stroking there (not that there's an ego to stroke to begin with), I'm a militant anti-theist that honestly believes the world would be a better place without (most) religion (but that's a whole 'nother kettle of fish I don't want to open here), so it's not for spiritual reasons, and as I said, I actually tend to get mad at (or just react dimmissively towards) people that try to treat me like I deserve some kind of recognition for my actions, so it's not for real rewards...
*sigh* I'm just going with '*shrug* whatever,' as that just so happens to be my go-to response to most people when they thank me for my help, and my actual response to this thread after thinking about it for a while.