When you are raised by a man who wears a mask that hides everything but his eyes, your perception of facial expressions becomes twisted. Kakashi had no idea that he perceived his father and himself differently than everyone else until one of his classmates asked him why he never smiled.
"I do smile." He had retorted.
"No you don't. Your eyes smile, but your mouth doesn't." Kakashi watched the other children after that, watched the way their mouths moved. It had never seemed important before, you could read everything about a person from their eyes, the mouth was just for making noise. But they smiled differently, in a way he didn't understand. When he asked his father - confused - how he could learn to smile, his father had smiled at him in a way he understood - with his eyes - and said that there was no need for everyone to express emotion in the same way. When his son passed the genin exam at an age previously unheard of he gave him a mask of dark material, and slipped it over his head with a serious air.
"Now you can smile however you like, and no one will know." He whispered, as he took the forehead protector from the examiner and tied it carefully around his forehead. "That is a ninja's right."