I guess most of the reasons involve the man feeling like he is not getting something he wants or needs from his relationship. This feeling certainly may not always be accurate. I'm sure that many times men think they will receive something they've been missing from another woman, but they are then disappointed. A man may think that his wife is too old, so he will try to find a younger woman. Sometimes he may find that his feeling have faded not because his wife is older, but because he is older. Another thing that can play a big factor is experience. With a long-term partner you gain a long history. This history usually involves ups and downs. When you have a new partner, you haven't managed to have any negative experiences together yet, so it may seem better at first.