This is going to vary from person to person. Yes, people hate others who are similar to us that represents all our fears, regrets, failures and so forth. Yes, competitors sometimes hate competition that have a high chance of standing in their way of winning or of fame. People hate others for how they look, how they talk, what their long dead ancestors did, what region they are born in, their ideologies and random beliefs, for having something they don't or hating others for simply making noises when they chew their food.
In short, there is no limits on the many reasons why someone hates someone else. Its infinite. Whatever you can think of, there is someone out there hating someone else for that reason. The reasons will range from being something understandable to something highly illogical.
I would prefer people put aside assumptions, start thinking of others as individuals, get true facts about others and over all start being a little bit more reasonable, understanding and compassionate. Hate blinds you and you begin to falter the more of it that builds inside you. The oceans of hate are overflowing and the cities are being flooded by it. Like the hippies say, let us make love, not war.
With all that said, we are humans and this is a human emotion that is in us all. We can choose to control it and understand it or let it run rampant and control us.