The immense satisfaction when someone asks you about a topic you're passionate and knowledgeable about and when you start info dumping, instead of stopping you or tuning you out, they actually listen and ask questions
I'm so used to people not caring that even with people I'm close with, I hold myself back because the usual reaction is either "Eww, maths" or just a general unwillingess to engage with anything outside their box. So when someone shows genuine interest, it's like Christmas has come early