I'm not sure why we are unfaithfull. Perhaps it has to do with our sin nature and our rebellion to submit; to be content with what we have. As men we are visual, and tend to pick up on the the things that we perceive pleasurable. Many times we admire what we don't have, without realizing that what we do have is more valuable, and complete. Maybe we are unfaithfull because we are unhappy with ourselves and don't know how to change, or haven't submitted an area of our lives that the Lord wants to heal.