Altruism IS in one's best interest. It makes you more likable to other humans. People are less likely to hurt someone seen as charitable or nice (see: Fred Rogers, whose car was returned when the thieves found out who he was), and I'd argue that nice, generous people probably are more often chosen as mates than stingy, mean people.
Most people aren't thinking of that when they give money to charity or to a homeless guy though. They're following a base instinct that tells them it's right to give money to a particular person or cause. We think it's noble to give up things to help others (despite it being an instinctive behavior) because it's beneficial to us as a species and as individuals to value altruism.
If you don't want to give up anything to help other people, that's fine. Give up the benefits of society and go live in the woods. Hunt your own meat. Build your own shelter. If you're attacked by wild animals or other people, don't bother screaming for help. No one will come. That is the reality of selfish living. And that's why there aren't many humans who want to live that life--back before guns, the odds of survival were not great.