Well, without it we wouldn't be here. But as EricNau said it's not something we're forced into (most of the time).
I also find it quite funny that apparently, by doing something charitable we're also completely selfish. I've had a great time on this planet so far and I can't wait to have kids and show them this big wide world, to look after them in their young years and have them look after me as I get older. To do what our bodies were developed to do (assuming me and my girlfriend are able to have kids).
Just as when we used to have dogs. They needed a home and we could provide a home and showed them all a good life, which in turn was our reward.
And maybe I'm different but I work for the friendships I form, for the satisfaction of doing a good job, for the feeling of being alive compared to being sat at home all day. Even the money I make is going towards paying off my student loans and saving up for a place for me and my girlfriend, it's not like I'm a super selfish person who works only to buy the latest electronics and clothes and such.
Just seems that some people are judging others by their own standards. Note: Doing something good doesn't always mean your selfish