I have been a "mapper" since the earlier days of Half-Life. I was never one of the greats. I had always wanted to create a map that would propel my name to Half-Life celebredom and see my bsp smeared all over the server lists. That was not in the cards for me. My maps were either proof-of-concepts awkwardly trying to push the boundaries or overly artistic with no substance.
My five year old son has been playing Gary's Mod recently. Since he took an interest in it, I loaded one of my old (2007ish) HL2 maps into it for him to play. I didn't expect him to like it. It was more of a, "Hey look, your old man used to do this stuff!" But days later, I was walking past the computer and he was playing my map. I asked him, "You actually like this map?" and he replied, "Yeah daddy, its fun!"
It was a very heartwarming moment for me. I never made the map knowing that over a decade later my son would be the one enjoying it. I felt like I had finally got validation in the best way.
I felt a bit like this when my brother and 2 cousins started competing, seeing who would speedrun my game prototype the fastest.