You choose people that make you happy in some form and hope that grows to more or that the time that passes while you're together isn't bad.
I've had my own share of decreasing happiness and while I wouldn't call them bad people we weren't good for each other
And I'm sure that's the way it is with her exes too
But additionally, some of that conflict brings out the worst in people and as her friend I'm only privy to the initial happiness and then the fall out and in that, they've only been trash to me.