Well, my grandparents marriage failed because of sex. My grandmother wouldn't put out unless my grandfather bought her something new, and then she'd just lay there. Though they did have 5 kids. They were old school too, the wait until your father gets home kind, my grandfather hated that.
A friend of mine says it is because her husband didn't love her anymore, and he took off to meet with another woman he met online play WoW. I think it is more because of her overbearing nature and constant reassurance she wants. After six months of supporting her while she went through her drama I was getting sick of her and understood why her husband did what he did. He must have felt trapped.
I don't know about my cousin. When she was in the hospital for an operation her husband only came to see her once, and told her not to come home until she could do her wifely duties. So after she was released she stayed with her mother to recover. In that time she decided not to go back home to him. Rumor has it though that she might have had another male friend on the side, her own mother let slip information that supports that.
Another cousin of mine, he husband ran around on her and was a dirt bag. He came off like such a nice guy at first, but fooled everyone. Her mother married my uncle when they were only 16 because her mother was pregnant with my cousin. They got divorced because he was a dirt bag too and forced her to do things I won't mention. He later remarried 4 more times. One of his wives gave him everything, her family was rich, he just used her. She ended up in psych ward when he left her. His 3rd wife found out about what he did to his second wife and she left him. He's still married to his 4th wife, but that's probably because he's scared to death of her father and brothers. If he ever screwed her over they'd hunt him down.
So...like ted said there are several reasons why people get divorced. Some rush into something before they really understand what they are getting into. Some do it because they are careless and get pregnant and "its the right thing to do". Others soon realize that the person they married wasn't the person they first fell in love with.