Sitcom rules dictated that Chandler and Janie would've eventually wound up together.
I go back and forth on Chandler and Monica. In a way, it came from nowhere, despite some later retroactive continuity (Monica's revealed "crush" on Chandler).
Chandler and Monica as a couple also cemented the decision not to expand the cast. They'd even later lampshade the fact that the "friends" didn't seem to have any other friends.
For me, it's an interesting contrast to, say, CHEERS & FRASIER, where the cast grew beyond who we met in their pilots (esp. on CHEERS).
