How come they don't treat everybody the same way?

There's plenty of things about my whole situation I can't make sense of, but this is one huge part. I'm not going to generalise because I'm guessing they don't all behave the same way, but I'd really like to know...

Like I've said before, in my case he didn't treat other people (acquaintances, mutuals friends, and surely his own friends) the way he treated me. He was "normal" with them, and honestly I wouldn't have associated the word "narcissist" with him before he got rid of me out of the blue. We were arguing a lot but that certainly wasn't new, he was already being selfish and inconsiderate of my feelings, it's not like he went from being an angel to an asshole overnight, I mean I'd even asked him several times why he would treat me, his best friend, differently from his other friends, and I didn't get a real answer. It was just "the others don't react the way you do/don't do the things you do". I'd never hide the fact his actions or reactions hurt me and he'd act like it wasn't a burden but I see it more and more this way now. He'd keep acting like someone who wanted to fix things but it never, ever showed. He'd say things but his behaviour didn't change.

If our mutual friends needed help with something, he'd help them. Sounded like it was a given he'd be there for them since they were his friends. He also helped me, although I did sometimes feel like he didn't really want to, but as soon as it was about us and confronting him on his behaviour, the helpful energy disappeared. It was like he took everything I'd say as an attack and he didn't even try to see where I was coming from, as if I had fun telling him to stop doing this or that, to start caring about me...?? As if I enjoyed arguing and facing a wall?? And a lot of arguments weren't even over important stuff, but he was so dismissive and unwilling to consider me that it would snowball and I'd end up feeling even worse. He'd even tell me I "saw things that weren't there"! Honestly it was like nothing could touch him, whereas I was the one being sensitive over everything. So yeah, the others didn't have this issue with him because he didn't fail to consider them, maybe they didn't give as much importance to certain things as I did but I feel like a true best friend should CARE how they make you feel and not basically blame you for feeling the way you do? I mean, he did end up telling me I was the one hurting myself, not him, so 🙃

I really don't get it, what is it that made him/people like him (which makes me sad to type cuz I thought he was one of a kind, I don't think anybody could understand how special he was to me) treat everybody else decently but not me, at least not forever? Why was I the unlucky one, out of all the people we knew, seeing that dark side?