“Will-They or Won’t-They” is the bread and butter of many TV dramas (and some sitcoms—I’m looking at you, Ross and Rachel). For some, it’s built right into the premise (Bones, for example), whereas other shows have it in the background. For a while anyway.
A glossary:
Shipping/shippers: nothing to do with boats, this refers to a shortened form of “relationshipping,” or “who you want to get together.” Viewers can “ship” characters on a show, which usually means they want the characters to get together.
OTP: One True Pairing. A viewer’s favorite ship, either for a particular show or all of their fandoms.
UST: Unresolved Sexual Tension. Something networks absolutely love. It’s the tease that they hope to capitalize on, but they of course can’t let their leads get together because of:
“The Moonlighting Curse”: The source of the rule that, when the lead characters on a show get together, the UST disappears, and the show tanks. The validity of the rule is debatable; many people believe that it was not David and Maddie getting together that ruined the show, but behind-the-scenes fighting and a season in which their leads were absent half the time. Still, networks and showrunners are often scared by the Moonlighting Curse and the potential loss of viewership and, subsequently, money. This is the reason why most shows end up putting together the main couple in the season finale. Sometimes they’ll do so beforehand, then break them up for some reason, then throw them together when they’re cancelled. One of the reasons for this is the belief that it is difficult to find drama or comedy once the main couple is together—because as we all know from real life, once you’re in a relationship, everything is perfect and shiny and rainbows and unicorns.
Shows that aren’t primarily based on the sexual tension between their leads have been edging into the concept of putting their characters together. Grey’s Anatomy has been doing fine since Shonda Rhimes declared that Derek and Meredith were going to be getting together and staying together permanently (after fans got all up in arms about the constant breaking up). It’s seemed to find a source of tension that is not whether or not characters will break up.
Or, at least, those characters. Grey’s has a bit of a break in that it is a prime-time soap, and therefore has a lot of other romantic relationships to fall back on. Bones tried to distract their viewers with Angela and Hodgins, but the show was built around Booth and Brennan. Fans have been clamoring for years to get them together, and at some point the level of UST gets to be ridiculous, and the networks end up turning viewers away by trying to prevent the Moonlighting Curse.
(As a note, there is generally a certain percentage of viewers who Just Don't Care about the relationships in any given TV show. They're in it for the plot and storyline, and they tend to be most annoyed when the relationship--or lack thereof--begins to take over the show. However, the fans who do care about shipping tend to be louder.)