  How hard can it be to conceive such a simple concept? Words come out wrong in the heat of the moment, you say things you don't mean, and then end up saying things to justify what you know as taint, or morally wrong, but never admit you are wrong. People tend to look at things from an inside perspective. They believe that what they say and/or do is just.
Even if they know it as not just, they will overlook the fact and force themselves to believe that what they did, said, etc. was just, justifable, and will argue that their point reigns supreme. Imagine two people in love, living together, and happy. Now picture the tiny quirks among them that tend to get on the other's nerves. They get mad, fight, words fly, but it is fine the next day.
Do they now see that what they did was wrong? No. They still believe that they are right, but set the argument aside, rather than talk about it, or admit that what they did was dishonest. Sooner or later the fighting builds up. One attacks the other, the other fights back. They now just want to pick fights and prove each other wrong. They get disgusted with one another and soon one is thrown out the house. One says the other just made things worse and made things the way they are now, and vise versa.
I tried to tell them that they both played a role, but both refuse to listen. Adults are alot like children: they fight constantly. The difference here is a child is incapable of holding a grudge for as long as many adults can. 
