I’m sure it’s been said before, and whoever it was, I agree one hundred percent (or to the max, if you prefer). All great stories are about relationships; in the same way that life itself is about relationships. They are about how different people relate to one another, and themselves, under changing circumstances.
They are about the things that make us who we are. Every person is different—his/her own separate part of humanity, and every person is the same—united by things we all share. Beliefs laid on us by society, behaviours no one can escape, events that no one can foretell, and the overwhelming and underlying uncertainty of all our lives. Great stories are about how we react to these, and all other parts of the world, with action or inaction. They tell us about life, and about ourselves. They present characters whose little joys and tribulations, we can understand, even when they don’t make sense. Because the world itself only makes sense up to a certain point, and beyond that we only convince ourselves things make sense so we are not lost in the vast.
Great stories do not judge the characters and actions, but allow great audiences to decide themselves. They do not tell the whole story, but leave room for it to stretch and seek. Great stories are given to us in a way we can unwrap them how we see fit. They are insightful and inciting companions, not leaders or directors. And in that way they show, but do not tell.
They make us love, and appreciate the world for what it is, or for what it could be. They clarify, or give to us, fragments of our own stories, to appreciate or to tell.
And on a related note, American Beauty is a great story.