For me, it's in large part a matter of balancing the things I want to say with the way I want to portray my characters, as I largely write fiction. I try to consider the ways in which their thinking would differ from mine.
Most of my characters, for example, do not read quite as much non-fiction as I do, or at least not for fun, so they don't have the wide general knowledge I do. Or they don't value reading at all. But a voice that disdains reading isn't quite the voice I want to write with, thus the careful balance.