Light Side and Dark Side feels a lot like surface level branding to me, maybe it is the fact that the Force is so vast and unending as a concept. So much so, that to truly comprehend it, you need to see the inner workings of the universe itself, but the ancient conflict that goes between Jedi and Sith, in simplified terms is.. How the force should be used, well that's my understanding of it at least.
Light Side, Jedi, focus on letting the force flow through them, and let themselves flow in it, like a river. Choosing peace and serenity over what's practically a fruitless endeavor like attempting to shape future in the way they want it, or chase ghosts of the past.
Dark Side, Sith, reject the notion of 'destiny' or dice that has been already cast, and choose to use the force in their own way, forge a destiny that's their own. Rather than letting go of the woes that befall them simply because 'it is what it is'. Going against the flow of the river and attempting to redirect the flow in their own.
Light Side may be the 'natural' way to 'use' the force, but Dark Side being 'unnatural' doesn't make it any less valid when you consider the implications.
That being said, I do think Grey FS are a bit weird, you can't feel calm, but be passionate at the same time, so part of me thinks that the true key to understanding the force is to know that Light Side and Dark Side is much like a state of mind.
Light Side is way, way more difficult to obtain and preserve, it is the serenity and calm, actual good mood if you might call it.
Dark Side is when everything goes into the deeper end, it's easier to give into stress and despair, rather than to work towards triumph, knowing the strife you have to go through.
There's no golden way to use both sides, as remaining on tipping edge between one or another would probably drive you insane. Simple fact is that you will fall to the Dark Side, like you do in real life, your fear and anger WILL overcome you.
But push foward, and see to the end of it all, and you find peace, and tranquility. The Light Side.
I can also be completely wrong, I just really hate for it to be good vs evil all the time. :<