Let's talk of romance and television characters.
There's a rule in place for most television characters. If they are the show's lead, then their relationship status rarely changes. If they begin the series single, that's usually how they end it. Same rule if they are married. It's a rare sight to see the lead of the show experience a major change in relationship status Show-runners like to keep the lead static, often out of fear that the show would change too drastically, and thus alienate viewers, were there to be a huge change. So the lead stays static.
That's why you develop the relationships of the tertiary characters.
The supporting cast will go through very significant changes, and that's what keeps the show fresh. Take the show Malcolm in the Middle (whose star has been in the news recently). While most everything about Malcolm and his immediate family remained the same, his oldest brother Francis saw dramatic shifts in his life. He was in military school. He moved to Alaska, got married, and found himself working at a dude ranch, and many other plots.
A really fun example is Cheers. The lead character, Sam Malone, pretty much stayed the same the entire 11 year run. He remained single and working at the bar. Tertiary character Frasier Crane, however, had many changes. He met Lillith, got married, had a baby, and got a divorce. That's the kind of changes you don't see in the main character.
Which is what happened when Frasier became the lead. Throughout the 11 year run of Frasier, he remained single the entire time, He would have short-term relationships, and a few potential long-term romances, but they would always end. The writers made sure to keep Frasier out of any long-term relationships until final few episodes of the last season. They didn't want to alter the dynamic of the show with too many changes.
That's what Niles and Daphne were for. Niles carried a torch for Daphne for seven seasons. Had he been the lead, that love would have remained unrequited, because the entire nature of the show would be building upon the sexual tension between the two, like Mulder and Scully, Booth and Bones. Since they were tertiary, their relationship was allowed to be consummated.
That's why a good series has solid supporting characters. They are allowed to change, have relationships progress, have children, and go through major life changes. That way the audience feels like the series is progressing.
Very rarely will you see a main character have huge character development, for fear of jumping the shark. Sometimes the show-runners will try to have it both ways with the classic called-off wedding plot-line. It seems that the character is progressing, about to make a huge life-change, and then they don't go through with it. The next season sees everything going back to normal, aside from a few throwaway lines about the canceled wedding.
You might then bring up the show Friends. Monica and Chandler got married, Rachel and Ross had a baby, there were life changes galore. However, the show had no leads, it's an ensemble, so the rule doesn't apply.
An interesting development in the Lead Romance is the show Cougartown. (I watch it, it's a great show, shut up.) Jules begins as a single divorcee, and now she's in a long-term relationship with Grayson. What makes this show able to break this rule is its shift in tone. The first few episodes were about Jules dating younger men, but the show then shifted in tone, focusing on the relationships among the cast. Thus, with a tonal shift (for the better) it was able to shift Jules into a relationship and get away with it.
Personally, I'd like to see more shows that allow the lead to have major life changes during the show's run. I get why they don't do it more, but it drives me nuts to have to wait until the series' end to see the main character hit major changes.