Well I can only speak for myself and my LDH on this...but for both of us it was a control issue. My LDH has been trying to desperately to make sure he wasn't treated the way his ex treated him that once we were married he shut down emotionally (because that's when things went downhill for him last time). For me being vulnerable meant showing my weaknesses and allowing someone in meant they could hurt me. Couple that with a bad self-esteem problem and bad body image and you have a recipe for disaster in a relationship.
