I would like to say, "Thank you..."

Oh god yes. Why can't we say thank you?!? It's such a common everyday interaction! Say thank you for gifts! Say thank you to bartenders, the pizza delivery guy  and vendors when they give you your order. Say thank you for being comforted when sad/uncomfortable, say thank you for being calmed down when scared/angry. (Slightly off topic, let a sad/uncomfortable sim ask to be comforted! Why do I have to switch household members to tell another sim to comfort a sad sim? Let my sad sims ask for a comforting hug! This game is lacking in so many basic common interactions.)

Make saying thank you for bigger things impact the game. Friend comforted you when sad? Give the sims a sentiment about it! Comforted sim said thank you? Give the sim receiving a sentiment about being appreciated by their friends or something. It doesn't need to happen when you thank the delivery guy for your pizza, but thank yous for bigger things like romantic gifts or a friendly adult teaching a sim kid to ride a bike? You'd assume sims remember that sorta thing for a while at least.