As always, neither Iruka-sensei nor Kakashi-sensei belong to me. Pity.
Kakashi knew a lot of things. He liked knowing, understanding, it gave him control. That's why he thought so much. Hatake's thought the way Akimichi's ate. All the time, without thinking about it.
His brain soaked up information like a sponge and stored it away, waiting for the time when it would be useful. Such as the time when Naruto had accidentally killed that snow rabbit. Kakashi had known that these animals change from white to brown in response to the change in daylength in the spring. So a white rabbit at the wrong time of year must have been kept indoors, away from nature's seasonal rhythm, to practice genjutsus on. And the time he'd noticed a flower with purple undersides to its leaves growing in a meadow. This is a feature of plants that grow in the shade, where the purple reflects the dim light back through the leaf. Plants growing in the open, in full sunlight, need no such help. That knowledge had clued him in to break the genjutsu that had led his jounin team far from its path, deep into the woods, and they'd been able to surprise the awaiting ambush.
He had no idea why he knew these things, where he'd learned them, but knowing and understanding was the basis of his genius. That was why Obito's eye made him so deadly. The sharigan eye provided the information, but it was the Hatake brain that stored it, analysed it, learned from it, turned it into a weapon to use against Konoha's enemies. No Uchiha had ever had the same power.
"You think too much."
Iruka put down the papers he was grading and crawled over the couch towards him. The teacher rested his head on his lap and stretched out, untied hair spilling over his thighs and knees like a pool of dark water.
Kakashi felt the emotion that the contact provoked swell in his chest and spread like a wave through his body. Why did it do that? What was it in this one man that caused this response? Of course he knew that the smell of Iruka's shampoo, and of his skin, was associated in his mind with the pleasure of sex. And that right now it was causing the levels of serotonin and endorphins and other pleasure releasing chemicals in his brain to spike in anticipation, but that only explained a small part of what he was feeling.
He lowered his book and looked into the dark eyes staring up at him. "What do you mean?"
The tanned face scrunched a little deepening the line of his scar, as Iruka snorted a little explosion of air through his nose. "Right now you're thinking about why I'm getting all pink because you're looking at me like that, and why your lips are starting to tingle because you think I'm going to kiss you." Iruka extended his arms to reach around Kakashi's neck, pulling himself up so that he could do just that.
Kakashi knew that the blood rising to Iruka's skin was a nonverbal communication telling him that the other man's emotions were stirring. He also knew that the blood gathering in their lips as they made contact caused them to become swollen, hotter, more sensitive. Giving a kiss passion that other contact, like a handshake, could never have.
He broke away reluctantly, reclaiming his tongue and his hot, swollen lips as they curled into a smile. "I'm just thinking about how much I love you."
Iruka grinned at him, eyes shining with laughter, love, lust, his hair fluffed and disheveled. "No you're not, you're trying to understand why."
Now blood was rushing to other parts of his body too. He could feel his heart pumping it, faster and harder as a cocktail of hormones were released into it. "That's easy, I love you because you're so damn sexy."
His lover chuckled and snagged the edge of his lower lip between his teeth, leaning back to expose the honeyed skin at the nape of his neck. "Then why aren't you in love with Genma? He's much sexier than I am, hell he'd even have sex with a venetian blind."
Kakashi cocked his head. "You're right, as a matter of fact I think he did once, in Cloud country."
The other's brown eyes became round. "Oh really, and you know this how?"
The blue eye became equally round. "Well I was there. As a matter of fact I was quite jealous, of the blind."
Iruka looked at him sternly, or at least as sternly as he could manage with such a huge grin. And with such a large bulge evident in his pants. "Ok, so if you were there and he was that desperate why didn't he have sex with you?"
Kakashi closed his eye and leaned back into the couch, pulling the other man back onto him. It sent chills racing across his skin, a sensation caused by the muscles at the roots of the tiny hairs contracting in response to the adrenaline into his system.
"He wanted to but he was afraid. He knew that if you found out you'd fly into a rage and kill him." Kakashi opened his eye into a narrow slit. "They're all afraid of you and your temper Iruka, all of them except me."
Iruka whispered hotly into his ear. "And what are you afraid of Kakashi?"
The pleasure of the sensation was cancelled by the image produced by the words, and the elite jounin's expression became serious. He stared across the room. "I'm afraid that one day you'll wake up and see me lying beside you and realise how much better you could do than an idiot like me. And that then you'll throw me out and I'll have lost the very best thing that ever happened to me."
"That will never happen." Iruka pushed himself back and pulled Kakshi's face towards him, forcing him to make eye contact. "It will never happen because I know what a wonderful person you are. That you aren't really such a lazy pervert, that you're kind, and caring, and loyal, and take care of your teammates and comrades. That you aren't really so mysterious either, you just wear a mask to separate yourself from people. Not because you don't like them but because you aren't comfortable being involved in their lives, but that you'd do anything to help someone who needs your help. I know everything about you."
Kakashi looked at him softly. "You've thought about this haven't you?"
Iruka's eyes were playful again. "I don't need to, it's obvious. Besides, I'm smarter than you."
Now there was laughter in Kakashi's blue eye too. "Oh you think so?"
Iruka sat up straighter, pulling his legs under him. "I know so. You see, you understand everything… but I understand you."
He pushed his weight against the taller man. Collapsing them both onto the couch, where he held him pinned, hands above his head as he ravaged his mouth with the ferocity of his kiss. Then he released his grip to let his hands wander. Kakashi allowed himself to melt into the pleasure of the onslaught. Yes this was part of it too. The closeness and comfort of being with someone who knew you better than you knew yourself, who understood and wanted to understand. But it was still only a fraction of the whole.
He gasped as Iruka finished unbuttoning his shirt and slipped his hands beneath the waistband of his pants. Maybe some things shouldn't be completely understood. Maybe sometimes it was better to not be in control.