Sims constantly 'Flirty' because of Sentiment!

I have had a varied experience with sentiments... some get flirty often, if they get the 'passion' sentiment (not sure how it's called in english). But not all my couples get this sentiment and not always it is a shared feeling.

I confess i love sentiments. It helps me define different kinds of relationships for different couples. But then maybe this is something i'm experiencing this way because i have been playing in rotation, so i get to play many different couples...