In the early 2000s I discovered swing music via Star Trek Deep Space Nine's character Vic Fontaine. I loved the sound of it, and after learning that Fontaine was supposed to be a bit like Sinatra, I bought a Sinatra CD ("The Best is Yet to Come", Reprise) while on vacation with my parents (I was still in high school) and listened to it on repeat for an entire week. Sinatra quickly became my favorite musician, period, and then I started reading books about his life and his friends' goings-on. In 2005/2006, only a couple of years after that vacation, I went through some personal drama (falling away from my parents' religion and politics, trying to create my own worldview) and became a big fan of Sinatra's brassy numbers -- stuff like "That's Life" and "I'm Gonna Live Until I Die". They inspired me to believe I would make it out of that period, and I began using the Sinatra avatar around that time -- 2006, I'd guess. Mistfit (a user here back in the day) created a version where Frank is wearing a Santa hat, and I used to switch to it during Christmas, but other than that it's' been 18 years using the same avatar. I also used it at a few other places like TrekBBS.
This is the reason I'm a Sinatra fan to this day.