Sims can have a limited number of Sims dearest to them that are under their protection and get very cross if another Sim messes with them, or optionally if they flirt with them.
Higher skill unlocks more new interactions.
This and many other such juicy scenarios get their own buffs!.
Try being alone in the same room with your crush!.
Show Number of Started Interactions with Sim.
Show Last Calculated Attraction to Other Sims.
Mark as Dream Sim (choose a Sim after whom to model your Sim’s ideal traits).
Force Recalculate All Relations to All Sims on Lot.