Y'know most people have this view of themselves which encompasses all their good points, and some bad points if they're not a ******* narcissist. Someone liking you means someone liking all that is good about you. Someone loving you means liking all that is good about you and forgiving your flaws. Either way it's some other person who appreciates the fact that you exist, and when it comes to love it's someone who really sees you and whose view of you coincides with how you see yourself, and so you're not alone in the universe (that's an oversimplification).
Now imagine that you have an entirely negative opinion of yourself. Anyone who get's to know you is going to hate you, and the better someone knows you - the more their view of you coincides with how you see yourself - the more they'll hate you. And this negative opinion of yourself might be completely undeserved, sure you can be a bit of a prick sometimes but you never meant any offence and you've never done anything that bad. And you think that if you could ever love someone and they could love you back (which always helps) then you might be able to love yourself. But you know what an unbridgeable chasm that is. You can't even let people get to know you a little because you're so sure that they're going to hate you and reject you. And you're really more afraid of people liking you than hating you.