Mark and Addison's relationship has always been my second favorite relationship behind Derek and Meredith. Addison was miserable and unhappy because she knew Derek loved Meredith. "He's just... the only people who don't know that Derek loves Meredith are Derek and Meredith." Mark admitted to Addison that he knew that Derek was in love with Meredith ano not her. "He’s not in love with you. He’s in love with that intern. He’s not even trying to hide it. Why would you want to stick around for that?" After Derek met Meredith and she decided to date both him and Finn, he saw Mark coming out of the bathroom of Addison's hotel room with a towel. Mark is the only man who truly understands Addison and what she wants. All the other men that Addison has been with will never understand her the way that Mark does. Mark is the only one who will ever truly Addison and Addison is the only one who will ever truly love Mark. Mark and Addison love each other and they will always love each other just like any other couple in love. Addison will never truly understand what it means to be in love if she's not with Mark. Even though Addison broke the sixty day bet she had with Mark, she was still head over heels in love with him. Addison knew that Derek would never truly get over Meredith. Addison's arrival was the catalysit that would cause drama between them for a while until Derek realized he was hurting Addison. She belongs with Mark because he would never betray her the way Derek betrayed her. They are madly in love just like Derek and Meredith.