My idea was always that Derek doesn't come from a wealthy background, but Mark and Addison did, so with out being married to money Derek never worked in his own private clinic.
Derek didn't drive directly home. He stopped at the Hospital and retrieved a letter that was in his office. It was a letter from one of his old attendings asking him to come be the head of their Nero department. It was a job of lifetime, it was everything Derek had worked towards, but it was on the other side of the country, in Seattle. When he first got the letter he almost disregarded it all together, but now, now he had nothing to lose. He'd be away from his mom and sisters, but he worked so much that he most of the time just talked to them on the phone. He was also being offered a lot more money then he was making in New York. Another perk was that Seattle Grace was a teaching hospital, the thought of molding young interns had its appeals. He retrieved the letter and then went home to his lonely dark apartment. His ex-fiance had taken most of the furniture and Derek hadn't bothered replacing much. His living room was almost empty, he kept his TV in the bedroom where also he had a small desk set up for his laptop.
Derek wasn't completely surprised to find Mark sitting at his small kitchen table when he walked in the door. A large bottle of scotch, that was about 3/4 full was sitting in front of him, he held an almost empty glass in his hands. He looked mad, but that was understandable. Most men would be if they caught their best friend in bed with their wives.
"I should never have given you a key." Derek said as he dropped his stuff at the front door.
"Do you want it back?" Mark asked as he topped up his glass.
"When I move I'll have to get it back." Derek walked into the kitchen.
"Your not moving."
Derek took the letter out of his pocket and slammed it on the table in front of Mark. As Mark read the letter Derek grabbed himself a glass and filled it. "Seriously, your going to Seattle?" Mark asked.
"I can't stay here."
"Why, because you were caught fucking my wife. You running Derek?"
"I'm not running, but that's a lot of money to give up" Derek sat down at the table. "Tell me why I should stay?"
"Family, friends, hell your from New York, you're like genetically engineered not to like any place that's not Manhattan."
"No that's you and Addi, Manhattan isn't home, Queens is my home." Derek leaned over the table and looked Mark in his eyes. "Why should I stayed, you said it, I fucked your wife, a few times. Not what a friend does."
"Do you want me to punch you, is that it. Trust me man I thought about it all the way over here." Mark refilled his glass.
"I want you to do something. If I was married to someone like Addi and caught my best friend in bed with her, I would hope to hell I would do something."
"I am doing something. I'm forgiving Derek. I don't want to loose my wife or someone I love like a brother."
"What if I love Addi?"
"She's easy to love."
"Stop it." Derek jumped up from the table. "I'm leaving. There's a chance I could be Chief one day. Here in New York I would never get the chance."
"Fine leave, if you think that's what you need to do, do it. You don't think I haven't noticed how hard you work, that's all you do Derek is work, maybe you can find happiness in Seattle."
Mark stood up from the table. "I should go. I meant it Derek I love you like I do my brothers."
When mark returned to his apartment, Addi was waiting up. She looked like a nervous fright. Neither one said anything to the other as she followed him upstairs to the spare bedroom. "Go to bed Addi," he told her.
"Did you hurt him?" she asked timidly.
"No. He's hurt himself enough," was all he said before he closed the door.