Why would he cheat on his marriage?

He had a devoted and loving wife for 20 years!! He left our church, home and even family to be with that tramp...Why do men do such awful and terrible things to their family..I loved with him..I don't think was the issue..we had fun together!!! No more homemade cookies for him!!! Why would a man do such awful things to a family that loved him and wanted him?? He left the comfort of his home for the little tramp..He always was into ...WHY??