I've also seen my sims spontaneously decide to flirt, kiss and make out - but never the first time! only after you've shown them how it's done...
Ness
That's how I thought it worked too. Until I had these two teens. One was romance, and one was knowledge. They were best friends, along with about 4 other teens. They all hung out together. I had lots of parties with them. Well, I invited these two teens over to a party thrown by another teen. They were friends, but no crush, never flirted, etc. They weren't who I intended for each other. Though they did keep whistling at each other, even the romance sim did it to the knowledge sim. Anyway, at this party where neither were controlled by me, they went at it, flirting then making out. And of course, bam in love since they were already best friends.
I thought, sigh.. Ok, they are so totally wrong for each other, but they were teens. It happens.
Then (this was before uni) I grew them up, all 6 of the teens, and they were some that grew up first, so were adults when invited to the last teen to grow up birthday party. And now as adults (and the crush/love gone since sims forget that when they age), bam they did the exact same thing AGAIN to each other.
So I just went with the flow. And they happily whistled at each other, and made out all the time spontaneously in their little house. Until I got the jump bug, and the maxis patch didn't fix it, and I just gave up on them and deleted the neighborhood.
Sigh. I miss those two.