Truth.
The depth of his works really comes to the surface when you recall the author's life. The Noldor seem like a simple tale about heroism leading to folly, but think; a high and noble race follows their king across the sea to do war in a vast continent against an evil foe, the orcs, who were once elves but became corrupted. In the process, the Noldor themselves suffer terribly, things don't go as planned, their champions are slain and the enemy has powers (dragons, balrogs) beyond their might. The war pretty well falls apart and eventually they begin to return home. But isn't it telling to consider what happened in Tolkien's lifetime? Raised in middle-class English morality, inspired by Victorian culture, joins the Army to fight the Kaiser and his invasion of France, everything goes to hell, the Germans have artillery and gas and machine guns, and although they finally win the whole thing is bittersweet and their greatness is lost in muddy fields across a foreign soil. And no doubt he learned the Germans were just like him, once.
"We were all orcs in the trenches." - J.R.R.T.
It isn't fairy-tale morality. It's a warning for the Noldor, whenever they return to Arda.