So, I was thinking, besides cheating or abuse, what is that last straw or that thing that made you snap and say, I'm done with this person or relationship?
Prolonged imbalanced in the relationship, and then a oneliner is enough to make me be done. Usually that oneliner is something which is opposite of whatever is in my own value system. I usually go fish for it as well, since I know it's there and I just have to hear it to be done.
It has been a long time but clinginess to the point of being dependent makes me want to run. Feeling like the other person doesn't care about me or he is trying to change me or control me are all things that would make me leave.
The most significant break up occurred after listening to a long standing favourite album of mine which basically sang the situation out from beginning to end.
IN my case, it was definitely when my SO would side with their friends over me. I've had an ex who had terrible friends (I'm probably most likely bias), but these friends were against us from the start, and my ex would always hold out hope for them than rather listen to me even when these friends were excluding us from activities.
Another reason to end things is when they depend on you too much to the point of being kind of creepy...and doing creepy/controlling things.