If I see they're changing or their will to change is consistent. But before that, I gotta separate experiences I've had with each person and trust them based on their merits and who they are. When the negatives start manifesting in other relationships that's probably a sign it's not healthy at all and by that point it's no longer about one person but on how I'm affected by it, funnelling through every other relationship I have.
So really, I trust people too much and not enough. I believe in people and their experiences and that they have a right to express the way they view things through their view, if someone can't express themselves properly with a clear line of communication, there will be massive issues that will build up unless addressed.
That's the thing, the person it involves may not know how bad it is until that line of communication is gone already, then there is nothing left to do but to move on. Build up, live life, find hobbies, find a new lease on life.
I don't really like the idea of ditching someone at their worst then coming back after they're at their peak. But I do acknowledge people need someone for the good times and the bad times, that may not be the same people. I've had to swallow that. It's pointless in dedicating when we don't have anything to give to prove or improve anything, if what has already been offered is not good enough even though to ourselves it would have been treasured gold to keep it.
I'm kinda talking on both sides of it there.