Here’s a pretty detailed rambling about the Force from George himself: https://www.youtube.com/watch?v=68dvgRT3Kx8
Lucas talks about the dark side as selfishness versus the light side as selflessness.
From video: “And you want to keep it in balance.” ^(following left/right arm movements reminiscent of a metronome)
Selfishness is dark side. The dark side itself is not the anger, greed, hatred, fear, jealousy, etc. - those are all just extensions of self-preservation and greed - manifestations of the extent you are passionate about what you desire. How angry or scared you are, defining how far you’re willing to go in your actions. What you want, how much you want, as correlative to what you’ll do to keep it. Serving only yourself and those desires - following a path paved by those intentions - is imbalance.
And it’s easy because at their basest level those feelings are natural, “biological pleasures” as Lucas puts it. You can want innocuous things, you can even want good things. That’s what makes it dangerous, that’s its power. Where the Light Side requires discipline to maintain, passion and desire is human.
Lucas talks about it as motivated from those tangible, quantifiable things - not occult magic. No side of the Force actually corrupts or controls you. Not any more than a child learning and pushing the limits of what they’re capable of, men earning money and status, or a politician with “power” in an authority sense. If anything, you corrupt it. The Force within yourself.
Lucas says that Force remains in balance when you follow its will. But he makes it a point in the video’s opening lines to say that you still have free will. A choice to reject it if you want to.
TLJ adheres to that point and tries to reiterate that almost to a fault (since a lot of ppl don’t like Luke’s arc in it. Or Poe’s. Or Finn’s. Even Rey’s for that matter). Luke talking about the “balance in nature between light and dark” is in a conversation all about the Force within oneself. The natural balanced energy that Rey could feel throughout the island as well as within herself. Luke specifically says this doesn’t belong to anybody, nor is it a power you wield. This entire point you brought up as a fundamental misunderstanding, is slavishly adherent to Lucas’s ideas.