It's an easy mistake to make: creating characters too much like you. As you develop in writing, you learn to build a more various cast and include some personality contrast. We're still writing from within our own experience, though, and unless we notice that our experience isn't universal, we just keep on replicating it. I only discovered recently that most people don't see music - so I have to comb back through my novel, checking that my characters aren't all synaesthetic. I'd just assumed that everyone sees the world as I do, including the music. An issue like that can just be idiosyncratic. When it comes to the experience and assumptions of our various identity positions, though, that matters more. We think we're writing a varied cast, but actually every character looks like us.
