Humans have egos. It is the way we are wired. If everyone did feel the same, we would be like boring clones. A difference of opinion can be either positive or negative. We were given the gift of free will to be accepting or non-accepting. It's just a fact of life.
I think it's that some people want the whole world to think the same way, and THEIR way. So they don't consider the fact that others have their thoughts and feelings and that we should have the freedom to think whatever we would like to. It gets very annoying, but we may be hypocrites by saying that - even I myself admit it. Sometimes we just don't pay enough attention to others' feelings and our actions, and we just let it slip.