Here's one thing I observed:
Last week, I was using my laptop, and I started a brand new game, with a brand new Sim, in Willow Creek. After the Welcome to the Neighborhood event, Travis could be counted on regular visits, along with the others from the BFF household. I'm convinced that's what they're there for in the game - to be the best friends for your Sim. A couple of weeks into the game, Summer came over in a flirty mood, and I decided to roll with it. She became good buds, with my new Sim. Well, two days later, Liberty shows up at the house. She was just in a normal happy mood. I think she was still at 1/4 green bar. However, as I was browsing the fridge for something to serve, I noticed some hearts being cued up. One was her complimenting my new Sim's appearance, and I think he had a flirt cued up. Also, as I was watching, they took turns taking each other's pictures for a while. It wasn't enough to give him a pink bar with her, but it is an example of autonomous "romantic" interaction.