Well from the guys I know it's usually because 1. They weren't really in love with you in the first place. They were using you for side candy. 2.) Maybe did really love you, but can't afford to have their heart broken again. 3.) Don't want to be judged, hear negative criticism, nagging, and how you pick out all their mistakes and short comings. They are not as strong as woman emotionally, spiritually, and usually need you to be patient, understanding, compassionate, and forgiving. 4.) just can't keep a commitment and it's to hard for them because of past issues. 5.) Some times you're not the issue, and it's so not about you, but them dealing with baggage.