It's not expected that you'll be suspicious of your girlfriend.
Doug follows his girlfriend Sonja because he's suspicious of cheating since all his previous girlfriends cheated on him.
Doug wants to decide if he should continue this behavior.
Doug realizes he's basically punishing Sonja for the actions of previous girlfriends.
Doug continues to stalk Sonja because he now thinks that women cannot be loyal by default.
Doug stops and instead spends more time with her rather than following her.
Doug doesn't have time to spend with Sonja because he pretend to be busy, so he can follow her.