Mark and Adena have a candid conversation with each other about their relationship. They look back to how they could have destroyed each other in the early years.
To summarize, Adena was threatened to be a person that had to have a perfect life. This could have caused her to reject Mark since he was a very flawed man.
Mark was threatened to be a person that would never admit failure. This could have caused him to bring Adena so much pain as he hid his weaknesses.