Having the Baby.
The budding love story is a go to for writers. Everyone loves it, and makes you feel emotions when they finally get together. Problem is, it has a natural path.
2 to 3 seasons to get together. 1 season of new bliss, 1 season of ups and downs, ending with a marriage proposal. 1 season of engagement ending in wedding. 1 season of new marriage stuff. Now what?
Married couples are boring. So what do we do now? Now it's time for the baby.
And babies are horrible on TV. People watch TV to escape reality, not hear a screaming child. So the dream couple has a baby and it's so tiring and so much work, but suddenly the show starts focusing on other characters, and then suddenly you know it's over.
The office was famous for this one. Everyone loves Jim and Pam, until the wedding, then who cares. They tried to force those feelings again with Andy and Erin, but you just can't.
Parks and rec luckily took a different route with Andy and April, but you can tell they were teetering on the edge, and in the final season everyone had kids anyway.
HIMYM had a worse approach because it wasn't that Ted was on the path, but rather Lily and Marshall already were and so kids came in earlier, and again change the entire show.
The list goes on, it is an official trope now