To become obsessed by a romantic interest, over-analysing the relationship, especially in the early stages. Focusing way too much attention on this person and letting it occupy your thoughts 24/7. Most commonly occurs in people who have too much time on their hands and not enough hobbies.
Single girl: Oh my god, I just text lover man and he hasn't written back yet! Do you think that means he isn't interested? Oh wait he wrote back, what do you think that means? Should I write back? I should wait 20 minutes first, right?
Friend of single girl: Girlfriend, you've got brain disease over this guy!