Here's a mega-inspirational quote from the one, the only Samwise Gamgee:
SAM: Even darkness must pass. A new day will come. And when the sun shines, it will shine out the clearer.
Wow Sam, really going out on a limb there. He believes that the night will eventually end and the sun will come up, just as it has, by definition, every single day. Don't call him a prophet, but this guy has quite the circadian rhythm. Oh… is that not what he's talking about. Is this whole "light" and "dark" thing some sort of crazy metaphor about good and evil?
Well, of course it is. You already knew that, didn't you? This is one of the oldest one in the books, it's just so dang intuitive. Light is good because in the light we can see, and things are clear and we are safe. But in the dark? Suddenly everything is hostile, even the tree branches outside our window, slowly scraping up and down the glass like the fingers of the undead (we used to have a lot of nightmares). The point is that light as "good" and dark as "bad" is such a basic metaphor, it doesn't really need explaining.
So instead, let's look at some examples—some dark ones.
Elrond tells Arwen, when she's thinking of marrying some mortal dude named Aragorn:
"But you, my daughter, you will linger on in darkness and in doubt, as nightfall in winter that comes without a star."
Way harsh, Dad. Darkness is bad because it's a nebulous, unknowable thing. It obscures what is good and causes even the most faithful to doubt—and the bravest of Shmoops to have zombie nightmares.
Here's more darkness-is-bad propaganda: Galadriel speaks to Elrond, saying, "In the gathering dark, the will of the Ring grows strong." It's almost like the dark has a physical presence here. It is something that can be gathered, or gather itself, a manifestation of evil.
And just to end on a happy note, we'll throw a little light-is-good quote your way. Gandalf tells Aragorn, "Look to my coming at first light on the fifth day. At dawn, look to the east." Dawn is the beginning of the new day when darkness begins to recede. It is at dawn that Gandalf appears with Éomer's riders and turns the tides of the war.