In the course of refining a general education literature syllabus, I've noticed a few common themes that repeat in the works I include. These recurring issues have mostly been unintentional.
Some of these common threads make sense. We read a lot about parent-child conflicts, and I suppose that is as close to a universal theme as you'll find--generational tension abounds in the history of Western literature.
Another common theme is "Ideal versus Reality." I do have a theory on why so many works involve some exploration of a fantasy, image, or ideal conflicting with reality. Fiction is fake, phony, not real. When devoting energy to making up stories, to telling of things that never happened, the writer may become keenly aware of the tension between fantasy and reality (or may feel driven to work out this tension in art). Many writers confront the fakery of fiction directly with metafiction, but even those that don't feel that tension, and so that conflict of an image against reality recurs in literature.
But one unintended subject I always find is insanity ("The Yellow Wallpaper," The Bluest Eye, One Flew Over the Cuckoo's Nest, King Lear, Death of a Salesman, Equus). I still don't have a coherent theory on why I fill the course with books about madness. Is it my own esoteric selection process? Or is insanity a very common subject of literature? And to either of those questions, why?