outside of work, he likes to relax and have fun since he works in a pretty emotionally taxing, high-pressure environment, so not!serious would have worked well for him. he also tends to work the night shift at the hospital, so between their two schedules and the fact that he probably wasn't looking to get into anything complicated, maybe things just ran their course? or if after a while, it because apparent he was going to take anything further (like he never would have taken her home to meet his dad even though he has dinner with him weekly, etc.) they just sort of decided they'd both enjoyed what they had for what it was but were both ready to move on? maybe they had a period of time where they didn't talk or really see one another after that but are friends or at least on the path to being friends now?