She slept with the professor
I kind of liked this movie until Emily Blunt's character lived with and slept with the professor even though she said when they kissed it "meant nothing". Then she's got a new hairstyle, and her ex is banging some 20 years old twit.
At the end, Jason Segel's character is so weak he still takes her back. It reminded me of the terrible ending to "Friends with Benefits" where the guy acts so weak and whipped. I don't think any guy would take her back.
This movie is also a good 35 minutes too long.