Disclaimer: don't own Parks and Rec or the characters, etc.
Chris likes to movement. He likes to run, bike, anything that gets his heart rate up. That's why he took the job. Ben took the job because he has something to prove. Chris, well, he took it because "on the road" was in the job description.
There's nothing wrong with Ann. In fact, she's phenomenal. She's sweet, intelligent, pretty. She's also content.
He's dated lots of different women. Most of them were marathon runners. Literally. And figuratively. Women who ran the corporate race until they were CEO's or women who sprinted all the way through their law firm until they became partner.
Ann was different. She worked hard and loved her job but she didn't mind coming home and sitting on the couch and popping in a movie. She was fine with a day off spent in pajamas while reading a book, coffee in hand.
That's what scared him. Ann made him want to stop running.