Hm-m-m-m-m... well... as a matter of fact... sometimes it doesn't. That's why the divorce rate is as high as it is. The fact that a relationship changes over time doesn't necessarily have to mean it goes sour. It just becomes different. But I think, especially for men, it can be difficult to accept that change & not start looking around. Men are genetically programmed to spread their genetic material as far & wide as possible.