Many reasons. They are bored. They feel unfulfilled somehow. Or they think another woman would be a better deal than the one they have. Or they want a woman who will better tolerate their habits. Or they think that if they don't play around they are missing out on life and adventure. Or they believe that the perfect woman, who will always be agreeable and never complain or ask them for anything, is out there, if only they keep looking. Or they want to have an affair as revenge for something you said or did. Or he feels that you don't appreciate him and wants a woman (usually younger) who will look up to him with big adoring eyes. Or they are insecure and want multiple women to prove to themselves and their friends that they are still studly and manly. They might be "married" to alcohol or drugs and prefer to hang with that crowd and its women. Or he is scared thinking he doesn't deserve you and has decided to cheat on you before you get a chance to cheat on him. Or he is a narcissist and does what he wants without keeping his promises or caring for what you think or feel. These are also the reasons that women get restless and look around. In any of these cases, these men or women are living in fantasy worlds, and if they won't grow up and get real, bid the fellow, or the gal, goodbye and good luck. If you are not sure, read the recent book How Not to Marry the Wrong Guy, by Anne Milford. It can change your life.