If the mean people in our lives were crappy 100% of the time, it would be easy to leave them. We would shrink from becoming friends with them or jump aboard the nope rocket in the early stages of trouble, and we would feel only relief when they are gone from our lives.
The problem is that very few people are evil all the time. They don’t wear villain costumes purchased at ForeverEvil. They don’t laugh maniacally and stroke their evil goatees while monologuing about their evil plans. They appear in our lives as People-Who-Would-Be-Awesome-Except-For-That-One-Glaring-Problem. They have potential to be awesome, and sometimes they are awesome, and they make us feel awesome, so we relax and let out that breath we’ve been holding in, and then BAM! They show their mean side, and we do a ton of mental work trying to reconcile the mean stuff with the awesome stuff.
Breaking up brings relief, as you lose the constant mental labor of managing the relationship AND the stress of being constantly disappointed and hurt, but it also brings grief. Shitty people who forget your birthday and give little backhanded compliments and gossip about your secrets sometimes give really good hugs, or presents, or are your favorite people to get drunk and watch figure-skating with, or were the sole witness to an important time in your life. The good times were real."