Hair
L didn't much understand Light.
He came to this conclusion one morning as they were in the bathroom getting ready. The pair were obvious polar opposites, you could tell that just by looking at them. L had heard the phrase opposites attract, but this was just ridiculous. The heart wants what the heart wants. He supposed. L already knew his strange behaviour irked his lover to no end. Light's behaviour simply interested L, but it made him realise how little he understood his lover.
One of the things which intrigued L most was his daily routine for his hair. When L got ready, he ran his hands through his hair to muss it up a bit and he was done. But Light took hours to get his hair just right. He would get up and shower, then towel and blow dry his hair before straightening it and using hair spray to keep it 'perfect'. L just couldn't understand it. He thought there was nothing cuter than his love with bed head.
So that morning, as he watched his love spraying his hair and being completely confused, decided he would help Light. He surreptitiously moved behind Light and as he put down his spray, placed his hands in his hair and messed it up.
All was still and silent as L admired his work and Light took everything in. Then slowly, Light turned round, eyes blazing red. He advanced slowly towards L, cracking his knuckles. "I'm not afraid of you Light." L chuckled.
"Oh I'll change that." Light growled, and L did begin to worry.
L lay on the floor, his nose bloodied, cheek bruised and sure every bone in his body was broken. "Be glad I love you, otherwise I would've beaten you and used the Death Note." Light said cheerfully before walking out of the apartment and off to school with less than perfect hair.
Ryuk floated into the apartment moments later and chuckled at L. "You messed up Light's hair didn't you?" L just groaned and hit his head against the floor as Ryuk chuckled at his misfortune.