I'm not sure I really know the deep rooted reason.......but I'm guessing it's my exH's mom........after she passed away (2 yrs. prior BD)......several childhood memories resurfaced. My exH told me after he left me that he was always "expected" to keep peace with all the siblings. He said he was tired of always trying to keep everyone happy other than himself. He also told me that he felt like he is codependent and blamed his mother for that also. He is a twin and I know it's difficult being a twin.....especially of the same sex. I can imagine there was always some competition between him and his brother. I have a female friend who is also a twin/girls.......and she has expressed how difficult it can be.
But, with all that said, my exH has also stated that he guesses he is just like his dad.......who left his mom, divorced and remarried.........the OW. They didn't have a good relationship for many years (exH was 17 when this happened). My exH made amends when his father was on his death bed. My exH also expressed to me that he has never learned how to process issues and let them go. I guess this is more than enough to cause a crisis.