A romantic comedy is not only a comedy with romance in it. A romantic comedy follows a specific formula: girl and boy meet, girl and boy lose one another, and then girl and boy reunite.
Typical story structure begins with the hero having a problem. In act 2, the hero can’t solve this problem, and this problem is only resolved in act 3. According to Billy Mernit, author of Writing the Romantic Comedy, the central question of the rom-com is not then “will the hero obtain his goal?” Rather, it is “will these two individuals become a couple?” Will the boy get the girl in the end?
As the audience already knows what the outcome of a romantic comedy is going to be (the boy does indeed get the girl), then, according to Mernit, it is helpful to create drama by making one of the characters have a negative desire.
This negative desire in romantic comedies usually manifests itself as commitment phobia in one of the characters. One of the characters just doesn’t want to be in a relationship, even if this is exactly what he needs, in a dramatic sense.
The best romantic comedies will thus have more and more obstacles being thrown at these two characters, which will get in the way of them coming together, and thus create the drama (and comedy!). One or both characters might already be in a relationship. They might live far from one another. Or there might be something big that each will lose if they follow their love.