I think it's just a representation of the normal anxiety that everyone feels in a serious relationship, but blown up for TV. It's hard to pull drama from a nuanced inner monologue a character is having about their feelings, but if you make them freak out you can do a 22 minute sitcom episode about them instead. It would also be really hard to make a TV or movie romance where two characters just gradually spend more and more time together and open up to one another, so romance in media tends to revolve around "flash points" like the first time one of them says "I love you" or the first time they have sex.