Why are people so mean

Why do people have to have so much hate in them? I just do not get them, you can not live forever being so unhappy. Just do not get it. I know i have had my fair share of downs, but i am not hateful towards other people. Just try to be happy just once, see where it gets you, it doesn't hurt really, and no one will know if you don't like it. But others have to live with you in this world you might as well try to be nicer to them along the way.