Here is a short article quoting Brad Pitt on why he gives to charity.  It is quite interesting.

Brad Pitt explains charity work commitment
By Frances Kindon Sep 23, 2011, 4:23 GMT

Brad Pitt has revealed why he and Angelina Jolie dedicate so much of their time to charity work.

The actor and his partner both balance film careers with raising six kids and supporting good causes around the world, and the 47-year-old said he wouldn’t have it any other way.

“I hit the lottery – the whole cliché of moving to Hollywood and getting paid silly amounts of money,” he explained to Parade magazine. “I’ve travelled the world and seen mothers and babies dying because they don’t have a 30-cent treatment that is available in industrialized nations. I feel like I have to share whatever I can. You’re culpable if you don’t act.

“You know, we bitch about raising taxes. I think the argument is that ‘it’s my money, I earned it, why do I have to pay for other people?’ I get very frustrated with that argument. I don’t mind paying taxes. I live in a country that gave me the opportunity to make money, and most people on this planet do not have that.”