If you are trying to get a Sim to like another Sim, and you want to propose, sometimes the game will respond "I Can't think on an empty stomach". Before the Sim arrives, make a meal. If it gets late, delete the doors, then they cannot leave.
Buy The "Concatenation Station", then have the Sims make a potion. Keep doing this until they fall in love. Note: You must make the Sim drink the potion.