He likes women in relationships

"It seems to me, Nathan, that you may have lost interest in her because she started to go out with you. You started flirting with her when you knew her boyfriend better too, right? Don't you think that maybe you wanted his girlfriend, whoever that was?"

Nathan took in a sharp breath, about to argue her down, but he began to think about what she was saying. It was true that he had started losing interest in her around when her ex stopped talking to either of them, and he had never been interested in her when she was single...

"Maybe you're right, Doc." he admitted. "So what do I do?" She was a doctor, after all, she should be able to fix his problem.

"Well, I might be able to help, but more likely than not, you won't like the solution."

"Hey, I'd do anything to stop feeling like this."