The first time it happened, Hermione almost jumped out of her own skin. Luna had quietly walked up to the Gryffindor table, and sat beside Hermione. She didn't notice her till Luna ran a hand over her bushy hair, and sighed quietly. The brunette had jumped in shock, knocking her knee against the table. Luna had kept petting Hermione's hair with her fingers.

"Luna, what're you doing?" Ron asked, looking confused at the two girls.

The blonde had smiled a bit.

"Nargles," she replied and as she stood up to leave, she turned to face Hermione and brushed the brunette's shoulder. "They're all over you."

With that she walked back to her own table. Hermione watched her go with a confused frown. "What a loony," Ron said, receiving a punch from his younger sister.

The second time it happened it wasn't as unexpected. The blonde had been sitting behind Hermione, watching the Quidditch match. When the older girl felt fingers in her hair, she turned around to see Luna with her usual dreamy expression. "Luna?" Blue eyes had met hers. "Yes, Hermione?"
"Your fingers are in my hair."
"I'm aware."
Hermione sighed impatiently.
"Luna?"
"Yes, Hermione?"
"Why're your fingers in my hair?"
"Oh," the girl looked like she was thinking about it. "I thought I saw a creature in it."
The brunette tried not to roll her eyes, and turned back around. After awhile she turned around again. The blonde seemed like she was in a daze.
"Luna?" The girl met Hermione's eyes and Hermione could have sworn she looked hopeful.
"Yes?"
"Why're your fingers still in my hair?"

The third time it had been almost expected. It was a sunny day, and Hermione had been sitting outside reading. Someone sat down beside her. Hermione waited until she had finished the page to see who it was. Luna. The brunette quietly observed the girl, her book was turned upside down, and she didn't appear to be reading, instead it looked like she was studying the page.
"What are you doing?"
Luna gave her a dreamy smile.
"I'm reading," she replied like it was obvious.
"You can't read when the book is upside down."
The blonde smiled patiently.
"Of course you can."
Hermione chose to let it go, and went back to reading. After a minute or two Luna started playing with her hair. The brunette glanced at her, and Luna smiled.
"Nargles."
Hermione smiled back.
When Ginny found them later, Hermione was laying in the grass reading, her head, almost, in Luna's lap. Luna was playing with the brunette's hair, smiling contently.
Ginny smirked knowingly.

The fourth time Hermione didn't even notice.
After not being able to find anything about 'nargles' in the library, Hermione went to the Room of Requirement. As she entered the room, she was greeted by a small bookcase and a cosy couch. She grabbed a book and began reading. When she was halfway through the second book, the door opened. She jumped up in shock, but calmed down when Luna Lovegood entered, a smile on her face. She said nothing, but sat down with Hermione on the couch. Hermione laid back down, her head now in Luna's lap. The blonde beamed and started running her fingers through the bushy brown hair. When the brunette glanced up at her, she felt her cheeks heating up. Luna looked down at her dazed, a small smile present on her face. Blue eyes met brown, and both girls blushed.
"How'd you get in here?" Hermione asked.
"Nargles."

The fifth time it happened Hermione expected it.
She and Luna were sitting in the Ravenclaw common room. The blonde was telling Hermione about the creatures her dad was writing about, and Hermione patiently listened, trying to keep an open mind. Neither of them realised the time, before Hermione had to get back to her dorm. Luna stood up to tell her goodbye, Hermione stepped forward quickly, and kissed the blonde. As the brunette pulled away, Luna grabbed her head, kissing her again. The younger girl ran her hands through Hermione's curls. Her fingers tightned around the brown locks, as the kiss grew more passionate. And when they pulled away, Luna smiled and whispered.
"Nargles."

Extra:
Luna was laying on a couch in the Room of Requirement, Hermione on top of her. Her hands were tangled in the wild brown hair of her girlfriend. The brunette's lips were brushing up her neck, and captured hers in a passionate kiss. Hermione's tongue ran across her bottom lip, and she opened her mouth to allow the older girl's tongue to enter her mouth. Luna ran her fingers through the other girl's hair, and played with the ends of it. Hermione pulled away with an amused smile.
"You really like my hair don't you?"
"Nargles."