I've mentioned before that I beta-read a LOT for various people.
Three of the most common problems I come across in first drafts are outlined in this post:
Do your characters get upset... and then forgive each other right away?
Have them walk away angry and fume for a while.
Does one character seem to pop up in every conflict-filled scene to work as a peacemaker, diffusing the tension?
Are your characters upset with each other because of some sitcom-esque misunderstanding that can be (and is) easily resolved?
Pump up the tension, give them something real (and really complicated) to fight about, and let them duke it out. Perhaps literally.
As civilized human beings, we are taught to avoid conflict. That jerk who cuts you off in traffic... sure, we get angry, but it's not like we follow him to his workplace and take a Louisville Slugger to his drivers-side door. The snippy receptionist who would rather talk to her girlfriend on the phone than help schedule your next appointment... we put up with it. We may tap our fingers on the counter, sigh and mutter under our breath, but how often would we lean across the counter, unplug the phone and say, 'Get off the phone and do your job'?
Unless you've already mastered the sadistic tendencies necessary to be a writer, having characters really fight is hard. Escalating the tension, the stakes, the emotions... this takes both guts and craft to pull it off. Especially when it's drawn out anger in an intimate relationship (and by that I include family/close friends/etc). Think back to some of the things you fought about that almost (or did) ruined that relationship.
How do you write about anger? Do you fall back into your own, natural civilized avoidance cycle, or can you truly get inside the head of a character who is furious?