First of all, the movie is about Jake, NOT the girl. She is a mere compliation of many girls he has dated. Snapshots of several failed relationships. Her thoughts are actually his, what he believes she's thinking about. When she starts thinking in a direction he doesn't like (or negative towards him) he interrupts, changes his thoughts. But I do encourage you to watch it again, focusing on him, leaving her as just a mish mash of his past but all the thoughts are really his. It will make MUCH more sense.
Review of I'm Thinking of Ending Things
