Hmmm...
Strange, then. My Sim has had Dina over, off & on since about a week after he was created, last month. She regularly throws out hearts, but it hasn't started a pink bar, yet.
If I were to tweak the game, I might like a separate "Affinity Bar". It could show, or not show. Both green & pink interactions could contribute to it. Basically, it would determine how willing two Sims would be, once they decided they want to start a relationship. The Affinity Bar would rarely cause any drama, except maybe with jealous sims. It might allow for flirty, affectionate, banter, without necessarily leading to smoochy awkwardness. If a Sim has a full affinity bar, with another Sim, it might make it more likely for that Sim, to try starting something. It seems like that might be a good balance between "Instant Enchanting Romance" & "No Autonomous Romantic Activity". A developing Affinity Bar would cultivate warm fuzzies, but PDA's would only occur, once Affinity Bar gets to about 50%. The Romance Bar gets triggered by PDA attempts, while the Affinity Bar is at 50% or greater, between two Sims.
Just an idea.