(A/N: None of the characters are mine. Posted without being beta-read or britpicked)
The phrase was murmured in his hair, quietly and muffled. Sherlock was draped gracefully across John and their breaths were slowing. It was the one anomaly in Sherlock's character, the one thing that never fits with the rest. Sherlock never apologizes and rarely expresses gratitude, except after the comforting lull of post-sex rest.
John took a closer look at Sherlock, watching his relaxed face, the slight smile and the very vulnerability of the expression that makes John reluctant to ask, why? Why do you say thank you?
"And now you are trying to deduce why I say thank you after sex," Sherlock murmured with a bit of affection.
Damn him. Sherlock was one of those men where sex will never dull his mind. But if he's offering an explanation...it's now or never.
"It's a bit out of character for you. If you ever say thank you, it feels like it was dragged out of you kicking and screaming. This is an odd exception."
"You're being imprecise again, John. It's not an odd exception at all. I say it all the time after sex, so it's a very common occurrence between us."
"May I ask why?"
Sherlock gazed into John's eyes, looking as if he was trying to find the right answer. The expression lacked that certain something that denoted a Sherlockian calculation was occurring. Instead, he had a relaxed thoughtful look, as if he always knew the reason but was trying to put it to words. It was novelty to see Sherlock being thoughtfully careful, instead of spitting out what came out naturally.
"It's not something I can properly explain," Sherlock finally said. He held John's gaze and continued. "It's interesting, you are the first person whom I've had sex with because I simply wanted to. Sex has usually been an experiment or it was a means to an ends for emotional reciprocity. There's no cynicism, maybe sentimentalism..."
"You thought what we did was sentimental?" responded John with a wry smile.
"Don't be obtuse," Sherlock replied irritably. "I warned you that this could not be put into proper words."
Sherlock closed his eyes and paused. John wondered if this was the best answer he was ever going to get. It would be alright if it was, Sherlock has already given John more than what he expected already. They shared a flat, their bodies and he had Sherlock's loyalty and devotion. John really can't complain about what he has with this man.
"It's the only thing that I refuse to analyze too closely, it feels too fragile for the rigors of examination," began Sherlock slowly.
He opened his eyes, taking a moment to study John's face. His voice is soft and tentative, a tone that John has never heard before. "There are certain things I know that I don't feel the need to prove. I know that we make love without ulterior motives, it never feels sordid or exploitative on my part and I don't need to do as many deductions and calculations when we have sex. It's instinctual, easy and comforting. I didn't know I wanted that until I had it, so I'm thankful for being gifted with something I didn't think I wanted. It's unexpected, which makes it precious. There are very few precious things in my life, I feel I have to take care of whatever this is between us is. I do so by thanking you, it instinctually feels like the right thing to do."
An odd silence descended upon them. With a huff, Sherlock shifted off John, pulling the blanket around his shoulders and turned his back on John, clearly embarrassed by his confession.
"See? I told you it was sentimental," declared Sherlock. "I am properly mortified."
"That's not sentimental."
"Oh please. I just made a heartfelt confession that could have come from one of those dreadful romantic comedies you periodically inflict on me. That is very definition of mortification."
"No, you silly emotionally stunted clot, it's a stunningly beautiful thing to hear from you. I really am touched."
"Now we're both being sentimental," retorted Sherlock, glancing over his shoulder at John with a prim expression. "We either end this with sleep or more sex. I don't want to bring in any more greeting card clichés into our bedroom. Words are inadequate, I know what I know and that's the end of it."
John pulled Sherlock close for an embrace and kissed his shoulder. They laid there for a long time. It is only when Sherlock is about to drop off to sleep that John whispered "I know that wasn't easy to admit. Thank you for sharing that."
Sherlock responded by stealing the rest of the blanket away from John with a "hrmph."