If you have the Close Knit lifestyle, then the interaction seems to come up when a conversation is sufficiently friendly - I don't know if it's "pleasant conversation" or higher.
Relationship level with the target Sim governs whether its a gain or a loss to relationship (green bar) - I have seen it go both ways.
It is tricky to unlock - not quite a hidden interaction, but definitely the sort of thing your Sims need to work up to in order to have it succeed.
Limited experience, because it's only come up in one of my saves, but to my best understanding, that's how it works.
