A series in order to be a series need to keep the story going indefinitely. They do this by treading water, the same water, over and over. Once they stumble on a combination that works, one that strikes a cord with the viewing audience, they keep recycling the same formula.
One such formula is the pair that just can't make it into relationship status. One or the other is already dating, married, mourning, or there's some other more elaborate reason why the two can't be together. And this sexual tension builds and builds between them. If the show lasts long enough the tension is maintained so long that I no longer care if the two get the hook up or not. Or if they do it feels like they are betraying the characters somehow.
Now it is not the use of sexual or romantic tension that I dislike, it's the misuse of it. As mentioned it seems to show up the most in television, or maybe it's just that much more noticeable in that medium. But I've seen it in books too.
Sexual, or romantic tension is like an elastic band. The longer it stays taut the more strength it loses. But including it in a story is a way to keep people interested in your characters and keep vibrancy in your plot. The trick is to keep the right balance, it is a delicate tight rope dance. Too much and a reader will be strained waiting for the charters to get the hook up but never getting that release which they (and we the readers) oh so desperately want and need.
Come on, snap it. You know you want to.