I never believed Don and Megan's relationship from the moment it started. They really are not right for each other. That being said, I think they are both pretty selfish people but in different ways. Megan is willing to take anyone down in order to get to the top as shown by her lying to her friend about the commercial. Don thinks all his needs and wants need be met before anyone, shown by his cheating on Betty repeatedly and with many women. Selfish people can never go together.