Before I started dating, I couldn't understand why couples who break up can't remain on good terms. Surely if you got along well before and during the relationship, you could go back to that afterwards?
But the thing is that just like with friendships, relationships don't always end on good terms. Sometimes there is mutual agreement to separate, sometimes only one person feels the relationship has run its course, sometimes one person does something awful to the other, the list goes on. Either way, as others have said, you open yourself a lot during a relationship, which makes you vulnerable. In exchange for that vulnerability you get intimacy and openness when things go right, but when things go wrong the feelings of hurt and betrayal are all the stronger, especially if the things you said and did in your vulnerable state are used against you. And as has said before, some people can be real jerks, including during break-ups, intentionally or not.
I was not perfect during my first relationship, but after we broke up my ex said a bunch of really shitty things to me, and he's never recognized that (ie apologized), and I just don't need someone like that in my life. We're friendly when we see each other at social gatherings, but that's about it.
For me, the reason I'm bitter towards my last ex is because of what he did. He decided to drop off the face of the earth without telling me it was over. I had to hear it from an ex friend of mine. This ex-friend told me that my ex-boyfriend didn't love me anymore and all that. I was so pissed because I would have much rather my ex have told me this instead of just avoiding it. I would have probably been less pissed.
Ugh, the whole "ghosting" thing has gotten a lot more popular recently, and while ghosting is great for casual flings or in cases of abuse, ghosting someone you were in a relationship with seems so cowardly to me. You have reason to be pissed.
~Psychic