[reuben_tate]

Taking the time to care and be considerate of other people is a form of social investment. By caring about others and their feelings towards someone, you increase the chance that you'll be pleased with the actions the other person does to/for you. If you randomly decide to tell your waitress that she's a fat and ugly, then you should expect to have very poor/slow service for the rest of the night.

In a long-term investment sense, caring about people allows you to easily make friends and build good, long-standing rapports with people. That way, when you need help, you have a network of people you can fall back on.

Anyways, this only answers the question: "Why should we care or pretend to care about other people?" As for why one should genuinely care, that's a bit harder to answer.