The rule is that on just about any TV series these days, whatever the genre, no matter if it's a comedy or a drama, every unattached main character must at some point become attached. There must be a love interest.
If that love interest can be another member of the cast, so much the better. It saves on salaries. You don't have to hire two new actors. The writers are spared the trouble thinking up two new characters who must be likable, a requirement that makes it highly probable that they won't be in the least---they'll be "likable."
Don't worry if the characters pairing up are unsuited for each other. Don't worry if there's no chemistry between the actors playing the newly coupled couple. Don't even worry if there's another character one of the lovers would be a better match for. Apparently audiences need to be assured that their favorites aren't sleeping alone.
Love is always a good thing. Hence, the rule.
When did this rule kick in anyway? It wasn't in effect when I was a kid. There was no rule that regular characters had to pair off like swans and turtle doves. None of my favorite shows had a love interest. Captain Kirk didn't have a love interest. Jim Rockford didn't have a love interest. Hawkeye Pierce didn't have a love interest. They had love interests, plural. Even into the 'Eighties, main characters on TV shows were unabashedly and cheerfully serially monagomous. MacGyver didn't have a love interest. Magnum didn't have one.
When the writers felt like writing a mushy story they invented a character that a guest star was brought in to play for one or at most two episodes before she or he got killed, betrayed the hero or heroine, or decided for the hero or heroine's own good that they had to part forever.
That's how they handled in on the Ponderosa. How many wives did the Cartwright boys go through. Little Joe alone filled whole cemeteries with the women he loved and lost to illness, bullets, or an Indian's arrow. That was the Bonanza Way and if it was good enough for Ben, Adam, Hoss, and Little Joe, it's good enough for Shawn Spencer...
In his comments, I noted:
Two thoughts on "Why romance": first, it's probably a logical outcome of shows observing that they have internal continuity; most shows nowadays, if not actually being serialistic in nature, will at least structure things so that characters grow over time, and previous exploits/adventures/cases/whatever can be reference on occasion. Thus, a viewer who tunes into a sporadic episode of, say, CSI can follow along, while a viewer who is a big fan and never misses an episode is kept going by whatever serialistic storylines the show has going on. Second, the focus on romance could come from the media's ongoing obsession with demographics. Maybe they just figure that romances that don't always end with a death or a plane ticket at the end of the hour will keep the women tuning in more often?
I'm going to flesh out my comments here a bit, though. First, in re-reading Lance's post, I see that he says that Magnum didn't have a love interest. He's referring, of course, to Thomas Sullivan Magnum, played ably by Tom Selleck. And Lance isn't quite right here. Magnum did have a love interest: his ex-wife Michelle, who turned up several times through the show's run. Early on in the series, we learn that Magnum married Michelle in Vietnam, and she was killed in a hospital bombing shortly thereafter. But then, in one of my favorite episodes of the show's run, he learns that she's not dead, but for various reasons he has to let her go, because she faked her death and married another. She comes back a few years later, figuring in the almost-series finale in which Magnum "died", and then she returned the next season only to be murdered for real this time. OK, the particulars aren't necessary, but what matters is that Magnum did have a love interest.
But I think what Lance is getting at is the series regular kind of love interest. Michelle wasn't there every week, on Magnum PI. That's probably a good thing, though; by keeping her appearances infrequent, it reinforced the notion that whomever else may come, Michelle was the love of Magnum's life (and, after her death, their daughter Lily). And each time Michelle showed up, enough information was given in the episodes' expositions that casual viewers who didn't watch every episode could figure out who she was. So when did we make the transition from "one love per episode" to "sustained lovers" on teevee shows?
Well, I'm not entirely sure, to be honest, but I suspect the change was fairly gradual. Lance also mentions Captain Kirk's girlfriend-of-the-week. (Of course, few of those could actually be considered "loves".) The movies would postulate that Carol Marcus was the real love of his life, of course; or rather, the second love of his life, after the Enterprise. But regular loves started showing up in sitcoms in the 1970s, right? I don't remember Richie Cunningham's beloved girlfriend being an official regular, but she was there quite a lot, right up until he married her. And then Fonzie himself got married. Laverne and Shirley had that Carmine guy. Part of these quasi-regular love interests were probably put there for realism's sake -- after all, how could they explain a high school kid in the 50s having a different girlfriend every week? That would have made no sense. And eventually the show went on long enough that having Fonzie constantly toting another name from his little black book around stopped working, so they came up with something new. (Didn't save the show, of course. It was gone pretty soon thereafter.)
With the arrival of Cheers, we saw the beginning of the extended sitcom romance, didn't we? Sam and Diane meet in the first episode, and flirt a while. They're on-again, off-again. Diane goes away and gets engaged to someone else (enter Frasier Crane). She dumps Frasier, back with Sam, and so on. Why did this happen? Well, right around the time Cheers was starting up, the grand prime-time soaps of the 80s were gathering steam: Dallas was huge, Dynasty and Knots Landing were rising. And before those, there was Little House on the Prairie, which had quite a lot of serialistic content itself. Even if that show didn't rely on season-long storyarcs the way the soaps did, they still had actual events that took place and had reverberations for a long time to come: Laura meeting Almanzo, the Ingallses adopting Albert, Mary starting that school for the blind, et cetera.
The change came on gradually, but I think the early 80s is really when episodic teevee started to reflect the notion that characters aren't just going to stay static forever; they get older and their lives move on. Surely the rigidly episodic nature of teevee shows occurred to people even during the 50s and 60s, didn't it? Did anyone ever wonder, a la the kids in Stand By Me, how come the Wagon Train never got anywhere?
Ultimately, I think Lance's "rule" that shows must always have a love interest is there pretty much because romance draws in the viewers, for the most part. Or it keeps them. And anyway, not every show has a love interest for the hero, does it? Jack Bauer hasn't had a steady one. (Renee Walker might be that, or she might not. This past season had some sexual tension between them, obviously, but nothing happened that went anywhere.) Earl on My Name is Earl didn't have one. I'm not as up on the show, but I don't think that House has one (unless you figure that he's bound to end up with Cuddy, but that's not the same as a constant love interest). Grissom had one on CSI, but that was on-again, off-again; I'm not sure that Mack Taylor or Horatio Caine have regular love interests on NY and Miami. (Mack's wife was killed on 9-11-01; Caine is in love, I think, with the cop who married his brother.)
In general, though, I think that Lance's rule stems from a general shift to a more serialized way of watching teevee. Law and Order excepted, there just aren't that many shows consisting of self-contained episodes anymore.