Sometimes I know the reason. I find that most people who go around disliking others are usually arrogant or they are reacting to something in you that they don't like in themselves.  Unless someone has a horribly off-putting personality trait, like they are extremely hot-tempered, I don't think there is much reason to dislike people.  People by definition are flawed.  No one is perfect (or even close to it if we're being honest here).  We should try to overlook the small irritations of the imperfections of others just as we would want ours overlooked.

I don't think the reason is important.  I would not let it bother me.  I do not live my life according to whether people like me or not.

If you are a forward person and it affects you--you ask the person concerned. If , on the other hand, it does not bother you -- ignore them. Unless they do you harm. Act on it by protecting yourself.

Why others like you? 

Why you do like some? and dislike the others? 

It's feelings, to me this think can't be explained in some cases. Other cases there are reasons, I have might had a bad day and had a go at someone they can hate me for ever, or on the contrary I did something nice to somone and they just start to like me, but I guess most of the cases it's a bout feelings and that part of who we are, like why you have nails at the end of each finger. 

My personal opinion though. 

-Clash of personalities-