People should treat each other nicely. People should respect others relationships. People should not cheat on their partners. People should...

The list can go on and on.

In an IDEAL world, yes, people SHOULD. But we all know we don't live in the ideal world. TRP has stressed this point before, but much of it needs to be reiterated.

I'm not promoting the idea of pure selfishness but rather, you should live your life in a way where you can get some benefit from what you do without having to feel like it wasn't worth it after the fact.

For example, if somebody asks you for a favor, don't do it expecting that that person will treat you any better or that they will return the favor. Do it because it makes you feel better about yourself or because you would do it unconditionally for that person.

Don't expect other guys to not hit on your girlfriend just because she's with you. Don't expect a girl to show you affection because you were "nice" to her. Don't expect people to reciprocate the "good" you do in this world.

You gotta live your life the way you want to live it. There are random variables that will affect your way of life, but as long as you control what you do to get what you want, you have more control over it than if you were to place your expectations on others.

No one owes you anything but yourself.