Melkor, autolatry.
In the bright pool that was the spirits of all his siblings, he could scarcely understand himself as an individual. It was only when he drew apart from them a little, drawn to the darkness by curiosity, that he began to truly understand that he was one, not a part of many, and he laughed aloud at the discovery; and it was the first laughter ever born into the nothingness. Discordant as all laughter is, but he was strangely pleased with the sound.
When the Music was made, he could not forget his discovery; that he was one and not a part of a greater whole, not if he chose not to be. And he thought upon how he had struck out alone, and lit the Void with the brightness of his spirit, and in the excitement of his knowledge he wove his own Music.
Discordant, some would say. But he loved it, pouring his heart into every note.
When Arda was made he loved himself even more; for he could make himself into whatever he pleased, set feet crashing down like mountains on Ulmo’s rivers to dam them, and the next moment twist down to nothing more than a mote of dust, catching the breezes and turning them around to confuse his twin. He did not mind the sighs and glares of his brethren, not when his own glory seemed to grow by the moment, a new aspect of his being unfolding and ready to be explored.
It was the cruelest blow, in the end, that he was locked to a single form; one that grew marred and aching with wounds that would not heal.
But he clung to the Silmarils, no matter how they burned him. When he was bathed in their light, he could see his reflection as it once was. Even if his burned hands ached and he could feel the roughness of the scar on his face, he could - not laugh, but smile at himself once more.
It was a threadbare happiness, but it was all he could afford, all he could hold onto after all this time.