It's of course not in general that was just my personal experience because I seemingly attract toxic people.
It's just that they were people I used to consider my best friend and my better best friend after that etc etc.. And some while into the friendship they just change, and no matter how forgiving I was or whatever I did or tried to change myself for the "better", it only ever became worse with them. We'd start fighting more regularly over increasingly mundane things, after a while our fights errupted into insults that get more serious with every other fight to the point where after the 50th fight or so I just didn't say anything anymore because it was pointless.
In my opinion the problem was that we became way too dependent on each other, for being only best friends, and loaded each other with so much of our own stress and trauma that at some point they just had to snap (I can handle stress pretty well because it's very much my standard emotion by now). I don't know how they saw it and I won't ask them because I know they'll just bombard me with insults or try to gaslight me.
It's only my personal experience, again. I hope some day someone can prove me wrong.