I believe that when we write, we write about ourselves, regardless of the subject matter we're dealing with. We write our own autobiographies in the mouths of characters who experience things we never will. It could be an adventure story about saving the universe, or a realistic, romantic drama. But our own personalities and life experiences inform everything that the characters see and feel. Mark Evanier put it more modestly - that he has a hatred for cole slaw, so if he was writing Daffy Duck or the Hair Bear Bunch, those characters would suddenly hate cole slaw too.
There is, and should be, a lot more to it than that. It's often a deeply uninteresting exercise to actually write our own life stories. We seek to move the audience - to take them on a ride that's exciting, emotional, funny - all of that. As writers we need to feel deeply, and treat the characters as being as fully-formed as ourselves. Our characters tend to manifest as facets of ourselves, and as echoes of people we have known. In theory I have very little in common with Marvel's the She-Hulk, but when writing about her shyness as Jennifer Walters, and her rocky relationship with her father, I realized we had much more in common than I'd thought. I wasn't writing my own life, but I was writing my own emotions - somehow writing my own experiences and feelings through the lens of a very different life.
Fanfiction, and work that's heavy with pop culture references, are considered the realm of the teenager, or the child. The mark of a writer who has yet to develop his or her own voice. Using an already-established character or world can be like training wheels. The writer already knows how that character speaks, and can imagine how that character would react in any situation. That's very good practice, all things considered, for learning how to do the same with an original character. The writings of a beginning writer, in this internet age, can be an incomprehensible mess of references to the films, songs, television series and other pop culture that have moved and inspired the writer to write. Referencing a song which had a huge effect on that writer, they believe that anyone reading the piece will have the same reaction, just hearing that song's title listed. Of course the effect is the opposite. Most people reading would lose interest at that point, having pop culture references thrown at them, and not get the intended buzz from that. They're likely to approach such a piece like walking through a minefield, and stop reading if those references get too dense, and seem to be standing in place of character development.
The writer, in this saturated internet age, must learn to throw away the pop culture references and look at what they're standing for. Why do we write? Why do these films and TV series and songs and books and comics and everything else inspire us? When I was a teenager, I was heavy with certain TV shows and films that I felt defined me. I wore them like a badge, and considered it a great victory to shoehorn a reference into one of my films that almost no one watching it would get.
That's the spark - the initial spark of seeing something that inspires that young teenage writer, makes you want to write. It takes a more mature writer to look at why those pieces were inspirational in the first place, and create something entirely original that has some of the same qualities. When you throw away the pop culture references, you're throwing away the training wheels and trying to stand on your own as a writer - Starting a process by which you might create something which could inspire someone else.
There is no shame in being inspired, but you can't inspire others by copying other art on a surface level. You have to dig into your own soul, your own experiences, even if it's just the fact that you don't like cole slaw. What do your characters want? What do you want? What do you value?
And those things that inspired you - why did they strike such a chord? What were the positive qualities, the realness and rawness? What was it that connected with you? How could you seek to strive for that in your own work?
Who are your characters? What do they want? And what are they fighting for?