I've got a couple but the one that has hit me in terms of writing was really heartwarming.
I started off writing online posting my original fiction on Wattpad when I was 14 (as many do), I rewrote it a lot and really tried to make it good, and for the writing level of a teen, it was, though still very bad and with a lot of the mistakes you would expect from a young fiction writer.
I eventually abandonned the account, barely posting because of artschool and work, and the account sat dormant for a good couple of years when I thought to maybe look back. I was surprised I had a new comment on that old story, from a name I vaguely remembered.
The comment made me cry so hard.
The commentor said that they had remembered my fic from back when I was still posting what ended up being the final round of rewrites, and out of curiosity had gone to see if it was still up. They had reread it and were now in the process of reading it with their younger cousin because they liked the type of fantasy world I liked and wrote about back then.
I was so touched because not only had this person remembered my story over 5 years later, but they still liked it enough to want to share it and use it to spend a nice time with their family. I don't expect to ever have something like that happen to me again (especially since I didn't expect it to happen to me at all) so I really cherish the feeling.