Bad communication, making assumptions (of what your partner thinks, how they would react to X or Y)
Less than complete honesty. Neglect. Apathy.
All a part of a (bad) breakup, I've learned.
I'm a part of that "club" of people who would rather talk things over calmly if there seems to be a problem in the relationship, instead of ignoring it, then potentially letting said problem grow.
Better to nip that shit in the bud, I say.
Makes for a more peaceful and enjoyable relationship. Or so the theory is.
In other words, I think good communication is KEY. Not necessarily talking 24/7 and not letting each other have some decent space as some have suggested(Honestly, was that the first thing that came to mind?), but to talk properly and honestly with each other when you do talk. Make compromises where compromises can be made, if needed at all.
There are two people in a relationship, each with their quirks, and each with their god-awful habits that annoy the crap out of the other person. If you don't talk about problems, or things that really annoy each of you, you'll be changing partners more often than you change your socks. And if that is indeed the case, and life throws you that many chances at experiencing some good 'ol happiness, then shame on you for wasting them.
My two cents.