Why should you be selfless?

Here's a secret: anything anyone does is ultimately selfish. If anyone does something "out of the goodness of their heart" or "for no reason," they're wrong. Most people are selfless because they're programmed to feel good when others feel good, and guilty when they make someone feel bad.

Some people justify this moral compass by claiming morality will get you somewhere in a supposed afterlife. Others do it because they think helping people is a way of getting people to like them and help them back.

Here's the big reason, though: one day, you will die. There is simply nothing you can do about it. When you die, all the effort you put into selfishness will simply vanish from existence. Humanity as a whole is a bit more permanent. By helping others and helping others help more people, you can have an impact that lasts essentially forever. If that's not a good enough reason, then I don't know what is.