Why does my ex hate all his exes besides me?

We dated last year, and even after the break up have remained close friends. We really loved eachother, and it was sad that it came to an end. We are young, so I suppose it was just how it was supposed to be at the time. He came out as being bi/gay and he has dated guys ever since. Out of all the relationships he has had, im the only one of them that he is close with. Like his other exes and him have FOUGHT A LOT, and it seems like they always end up hating eachother and really never talking to him again. I know everything about my ex and he knows everything about me, we have an undeniable connection that really is not explainable. We have been through so much bull, we should hate eachother by now but we are still great friends. Even though he lives in another town now, we still just are very close. Its hard to explain. He will call me a lot. He wants to keep me knowing whats going on with his family, im pretty close to his mom, and she is moving back and I am sure I will be there a lot. I just dont get it though. Why he keeps me so close, or tries too. And why he throws everyone else out and wont even look back, but with me he will always apologize and always come back. Its like he never wants to see me exit his life. Why is this? Why doesnt he do this with anyone else it seems like.