Why can't we all just get along?

It seems like in any era, we have people looking for peace but cannot find it. We have many different organizations all claiming to work for peace. Problem is, why is there still no peace?

And I am not even talking about world peace. I mean why can’t we all just get along? Why is there fighting and hating?

By the way, I am not a hopeless Pollyanna. I have accepted the cold truth long ago. I just want to start a good discussion on this matter.