Because they didn't really get together until Phoebus was willing to sacrifice his life to save the miller's family from Frollo. There was a bit of UST beforehand, but the moment where he saved the miller's family and she risked her life to save him -- that's where everything came together. And one reason I love that pairing so much. :3

I always thought they were intimate and playful but at the same time there was a certain tragicness in their story because they both almost lost each other at some point - when Phoebus nearly died and when Frollo nearly killed Esmeralda.

I think they are playful and have romance with a tragic background.
But most of all, I think their relationship is stable and everlasting.