It's not an obsession, exactly. Fixation would be more accurate. Strong fixation.
Spock groans softly, obligingly holding his own knees for the doctor as McCoy rubs around and around his hole, cupping his balls with the other hand, watching his cock swell green with blood. There's just something about the way Spock looks when he comes, that momentary state of abandon that's all the more sweet for its rarity. He likes that he can do this, and some possessive part of his brain is relieved that no one else gets to, that Vulcans are monogamous and Spock doesn't just want to come, but wants to come for McCoy. In fact, Spock's not really given to orgasm on his own steam, doesn't masturbate, doesn't seek it out. When they first started seeing each other, Spock was only interested in satisfying his partner, in kissing and sucking and being fucked, but never insisted on his own release. The first time McCoy coaxed out out of him, with fingers and kissing and low, dirty talk that made Spock come undone, he knew he had to have it again and again.
His fingers slide deep, working as a pair, opening and stimulating Spock as McCoy listens to the variance in breathing and heartrate and watches Spock's pupils dilate. For Spock, this is an intensely intimate practice, the hands being sacred, the fingers being as lips to humans. It's as dirty and gorgeous for Spock as rimming is for McCoy, and so he alternates between the two as means of bringing Spock to orgasm, with the occasional addition of a blowjob or a handjob for variety. Really, though, Spock prefers this, as it's difficult to have his cock touched when it's hard, almost like how McCoy's wife would complain when his stimulation got too direct. And McCoy's okay with that, with watching the rise and hardening of Spock's erection as a detached observer. It's almost hotter that way, though his eyes fling up to Spock's face the moment he hears that first groan of orgasm and he thrusts, rubs, fucks Spock harder with his fingers, watching him start to shiver and shake and then quickly replacing fingers with his cock, fucking Spock through those last few spasms. He holds Spock to his chest now, holding and comforting at odds to the rough pace of the fucking, and though Spock doesn't necessarily need this, he doesn't complain. McCoy presses his face into Spock's neck when he feels his balls start to tighten, hiding the emotion that he seeks out so avidly in Spock. And Spock never protests, just strokes the back of McCoy's neck with his fingers as he comes down and lets him take his pleasure in the ways he needs it most. McCoy will always be grateful for this.