I think the biggest reason people move on in general is that they don't have anything new to say about their lovers.
The seven-year itch is a word we all know. Many lovers can not pass the seven-year itch this hurdle. In particular, some men who have been with their girlfriends for a long time will feel that she has no attraction, and will move on.
In fact, two people in love at the very beginning of mutual attraction, because of a sense of mystery. You look at each other on the surface is very good, and then you will fantasize that he has a better place, always full of expectations and yearning for him, and then after you are together, with the gradual familiarization. His temper, his style, her makeup and dress up what you have seen more, compared to the beginning of the time together, you think she raised her hands and feet are elegant rich flavor. Then again, after seeing more beautiful women, you also feel that she is not beautiful, so you may move on.
Most women will be more dependent on their boyfriends as time goes by, and will transform this love into affection, and like into habit, while men will turn the initial heartbeat into boredom into a love affair.