He's been keeping a lot bottled up for a very long time, has been denying himself without admitting it, and now that she's opened up this part of him there's years' worth of energy and curiosity that needs to be let out. Which he knows should frighten him somewhat, or at least alarm him--but she likes this, she wants him every bit as fiercely as he wants her, and she's completely trustworthy.

Sherlock allows her to roll him to his back, smoothly and gracefully, sighing out a long breath into their kiss. The same sort of warm shiver he'd felt when she pulled his hair a bit earlier runs down his chest now, and he brings a hand up to curve around the back of her neck, just to have one more point of contact between them.