Why do people date? why do people cheat?
why do people break up?
serious answer

People date because they think they find interest in each other and they want to express it by hanging around each other more and going places together, maybe even alone.

People cheat because they either saw interest in the person they were originally with and weren't interested anymore, then didn't want to tell them, or they used them completely. (meaning they never liked them but pretended to) 

And people break up because one didn't think they wanted it anymore, they fought and thought it wasn't the best situation for them, or they aren't emotionally invested. Or, someone around them who loves them unconditionally didn't like the fact that they were dating to begin with.

I hope those are serious enough answers and that they help! :)