At some point you were sitting in front of a computer with a blank text box that said "Username:" to the left of it. You put something unique in that box. Why?
At this point, I've been "Link Skywalker" for fully half of my life. In 2001 my parents were finally going to allow me to make an email address all my own. At the time, the two most important things in my life were The Legend of Zelda video games, and Star Wars. Thus, my email address became Link_Skywalker_2001@yahoo.com.
Within a week I'd changed it to LinkSkywalker14@Yahoo.com, and I've used either "Linkskywalker" or "Linkskywalker14" to sign up nearly every account I've made since then. I feel like it's important to maintain a continuity with my username, so that people will always be able to recognize me. Although these days I prefer to go by "LS," because neither TLoZ nor Star Wars is really that big of a part of my life anymore.
AMUSING SIDE-NOTE: I am not the first Link Skywalker. I learned, about a year after I made the username, that there was another guy in the Zelda community who went by that name. He was apparently a huge asshole or something, because I actually got shadowbanned from a few forums because people mistook me for him.
But I own linkskywalker.com, so I'm the real one now.