I've noticed on many TV series where there's an "official couple", that unless they are married at the start of the series (and more often if they are already parents) it seems it takes awhile for them to become a couple. Usually one of them will be interested in the other while the other is either already in a relationship or isn't interested for whatever reason, and when being introduced to a show later in the shows run after the "official couple" became a couple it can seem awkward watching earlier episodes and seeing them not together. And in most cases it seems like the writers had been building up to the characters getting together, sometimes at the very end of the first season, or there might be an episode where they almost get together then don't and then get back together again.
Like on How I Met Your Mother, in the first episode Ted meets Robin and they go on a date, but she breaks up with him because he told her he loved her on the first date. But even though that turned her off they decided to stay friends and she was quickly part of Ted's main circle of friends (does this happen in real life, a couple who just met remain close-enough friends despite breaking up after one date?), and Ted continues hoping to be her boyfriend (while still dating other women). Then Ted gets a regular girlfriend, Victoria, who accepts a job or class in Germany, Robin invites Ted over while Ted is expecting a break-up, Ted lies about having broken up and they almost sleep together, until Victoria calls, and Ted breaks up afterwards and doesn't become Robin's boyfriend until a few episodes later, then they are a couple for the whole second season before breaking up. Then near the end of the next season Barney and Robin end up sleeping together but not becoming a couple, then for the entire next season Barney is attracted to Robin but they don't become a couple until the end of the season, only to break up after a small amount of episodes. Then sometime later they both become attracted to each other, on-and-off, while the other is in a relationship (or else one of them denies an attraction), only for them to have been engaged for the second half of this season.
And on That '70s Show, in the first episode Eric and Donna have their first kiss despite not being a couple, and it takes awhile for them to become a couple (and in the meantime Hyde also has a crush on her). They still seem to become a couple faster than most "official couples" who aren't together when the shows begin (and they had known each other for years). Then the third season ends with Eric breaking up with Donna, the fifth season begins with them back together (after the previous season ended with them wanting to be back together), then Eric leaves and sends Donna a letter breaking up with her, Donna and Roger eventually become a couple (after a bit of build-up), they break up in the last episode and I think Eric and Donna get back together once Eric returns.
And there's the other official couple, Jackie and Kelso. They start out in a relationship but for the first few episodes Kelso wants to break up with her but is unable to get himself to. Halfway through the season, Jackie thinks Kelso got her pregnant, but as soon as she finds out she's not pregnant, she breaks up with Kelso, they get back together by the end of the season, she breaks up with him in the second season after finding out Kelso was cheating on her with Laurie, the next season after Kelso finds out Laurie was cheating on him and apologizes to Jackie without expecting to get back together Jackie starts having feelings for him again and they eventually end up back together, only for Kelso to break up with Jackie when she kisses her boss, and a few episodes later they get back together until Jackie decides they need to get married, leading Kelso to run away from home, and in that time she and Hyde start dating (after she had occasionally had a crush on him during most of the times she and Kelso had broken up), and they eventually break up after Hyde cheated on her because he thought she cheated on him (I don't remember if they ever got back together after that, but they weren't together when the show ended). In fact one thing I didn't like about That '70s Show is the fact that almost every season either ended or began with one of the main couples (Eric and Donna, Kelso and Jackie/Hyde and Jackie, or Bob and Midge) breaking up.
And yeah, how come on many TV shows where there is an official couple, they tend to break up over the course of many episodes? Even if it's a show from back in the days when most shows had self-contained storylines this seems to happen. Is it because the writers want to try to get the characters dating other characters while not being broken up for too long?