Disclaimer: These characters don't belong to me, alas. I'm making no money from the use of JKR's ideas.
For some time, he had watched her.
Over the years, Headmaster Snape had observed a great many details about his Charms professor. He had absorbed her every smile and frown, her fanciful imaginings and bizarre customs, all in the hope that he might eventually solve the maddening, wonderful mystery that was Luna Lovegood.
He’d noticed that she’d hung a pomander of lavender and clove-studded onions in her classroom, to ward away Tringtillers. He knew that she rarely took house points when students disrespected her, but always took them when they disrespected fellow students. He’d watched her visits to the Thestrals, wandering out to the Forbidden Forest with bare feet and a bag of bloody meat.
He’d also noticed the silver of her hair in the moonlight and the way she twirled her quill when she was daydreaming during staff meetings. He knew the scent of her perfume—night-blooming jasmine and vanilla—and the glowing blue of her eyes when she was content.
He knew that her life had been filled with sadness, and yet she retained hope. He knew that she spoke only the truth because she saw no point in lies.
All these things, Severus Snape knew about Luna Lovegood—and yet on the day when she smiled at him and kissed his cheek and said that he had lovely eyes, he hadn’t seen it coming at all.
“I fancy you quite a lot,” she’d said. “Didn’t you know?”
There was, Severus decided, something to be said for a mystery that could never be solved.