“”We're born alone, we live alone, we die alone.
Only through our love and friendship can we create the illusion for the moment that we're not alone.
Altruism is a somewhat nebulous concept that involves "doing something for someone (or thing) else that brings no gain to oneself", and evolved over thousands of years as a social strategy when "every man for himself" didn't pan out so well. Atleast in social species like humans. There is some reason to believe that altruism may be the key to an advanced society.. We'll let you figure out the implications of that idea.
Some may also say that altruism in humans happens because you sincerely want others to be happy- whether you feel good about it or not. After all, people do give up their lives on behalf of others.
Cynically, the very act of "selflessly giving" is prima facie selfish in the sense that it makes one feel like a really awesome Christ-like helping healing person. Outside of deeply-felt Christian charity, altruism is usually genetically beneficent anyway.
 See also
- Ethical altruism
- Objectivism, for a philosophy which gives this concept (and generally speaking, the rest of the human race) the finger and celebrates selfishness instead.
- The Ant and the Peacock
- Reciprocal altruism