I think that I give, and am giving in part because I was programmed that way by my upbringing, specifically my mother. I was praised for generous behavior, and as a consequence, I believe I'm hardwired to get satisfaction from it.

Helping others gives me the following feelings, in different amounts, depending on the circumstance:

Happiness, accomplishment, nostalgia, fulfillment of duty.

I'm not religious, and I'm pretty much a moral relativist, so concepts like 'right', 'wrong', and 'karma' really don't figure into my mental equation, though I do believe in 'secular karma' if you will, which is basically the concept that if you're a shitty person, generally, nice people won't want to hang around you as much and the only people you'll be able to easily attract are other shitty people. Conversely, if you are good and kind, generally you'll find that other people of similar caliber won't be turned off by you and you're more likely to enjoy that company. Just common sense really, nothing mystical about it.

Also, I've always been pretty in tune with the emotional state of others. When I sense need or pain, I immediately associate and feel empathy and I think I kind of instinctively tend to that as if the pain were my own. This has less to do with charitable gift giving, and more with giving in an emotional interpersonal sense.