I started because my (then) girlfriend writes thank you notes all the time. I realized that thank you notes aren’t just for graduations and weddings, so the next time I was on the recieving end of a gift I made sure to write down my appreciation. It is fun to send the note, and the gift giver always likes to know that they chose well. Thank you notes certainly helped ease my transition from boyfriend to fiance—her parents are long distance, so they only knew me directly from one brief dinner encounter and several notes.