Major Offenders: The Office, Friends, Sex and the City, Scrubs, countless Soap Operas.
After months or years of increasing sexual tension, two leads finally admit that they love each other and want to be together. This usually occurs with a passionate kiss and a high pitched "Whoooo" from the studio audience. We at home get to believe that we, too, will one day find true love with the one hot girl in our circle of friends.
Then, tragedy strikes in the form of a breakup. The sitcom gets serious for a while, showcasing the tension between the ex-couple. The exes start dating new people and we get all sorts of jealousy and wacky misunderstandings, based on the fact that the couple is really still in love. Eventually then they get back together, only to do it all over again (if the series runs long enough).
Why it Works:
Romantic love is an emotion that supersedes all others--at least on television--and there's no better way to engage the viewers than by constantly giving it to them and taking it away again.
Also, the breakup stage allows shows to introduce guest stars to be the new love interests for a few weeks or months (Sarah Jessica Parker went through several in Sex and the City) which they believe will sustain the ratings until the next sweeps period, where they will reunite the beloved couple again.
Why it Shouldn't:
Repetition. This is the writers just going back to the same well for storylines again and again. Yes, we realize there is some realism to it, because we all know real couples that do the constant "get together and break up" cycle. You may recognize these couples as the ones who you constantly want to punch in the face.