Well, two words, but you know what I mean.

People try to maximize their own benefit. And if need be, they'll do it at each other's expense. In the words of Spirit, "they're all lookin' out for their woman, all reaching out for their dollar."
The workers want to be paid as much as possible. The bosses want to pay them as little as possible. That's why they're called "classes". Both sides are perfectly capable of driving the nation into the toilet just to get their way, and both sides will lie and cheat like a mad fucker in order to do so. That's why its called a "struggle".

The only fundamental difference between the two is that the bosses can do a lot more about getting their way than the workers. They have sufficient resources to be able to fight more effectively. Its got nothing to do with God, morality, economics, philosophy, ideology, principle, politics or any of the rest of that abstract crap. That's all just talk each side devises to justify what they want is right and what their rivals want is wrong. They are the consequences of class struggle, not the cause of it.

Its perfectly natural and normal to pick your side and fight for what you feel is right, for your fair share. I tend to pick the workers. Why? Not becuse I have some logical or moral reason for doing so, but because

1) I used to be one

2) There's more of them, and they suffer more.

3) As a rule, they are much nicer people.

EDIT: And let's not forget the most important reason of all:

4) The bosses can certainly afford it. The workers can't. Not any more.