It's the paradox of Anakin's graceful recklessness that drives Obi-Wan insane.
Because it fosters arrogance and ego and complacency, and he knows at this point he's Anakin's master in nothing but name. It plucks some deep chord of frustration and irritation that resonates deep in his gut, in a way that twangs uneasily at something he doesn't want to examine too closely, and he shoves it down until it sits low (too low, too low) in his stomach, solid and burning and demanding to be acknowledged.
So when the mission comes a hair's breadth from going horribly wrong—and Anakin is grinning smugly at him in that way he does—that fire of roiling emotion lights inside him in a burst of heat, and he tries desperately to smother it because Jedi aren't supposed to feel like this—
And then Anakin—his equal now in rank, and beyond his superior in power and skill—calls him (goads him) Master, a jab at their past that's condescending in tone but warm in affection, and the fire sets him ablaze.
He shoves Anakin against the wall of the bulkhead and snarls at him, and he sees surprise and delight and a challenge light in the eyes that never used to have to look down at him, and he reaches up and pushes down against Anakin's shoulders and is met with no resistance as his Padawan (his Padawan) willingly sinks to his knees.
He looks down at Anakin for the first time in years, and a part of his brain is sick at himself. What is he doing? But all that matters now is what Anakin is doing, because he's licking his lips and tugging the fly of his pants open with quick fingers and it's all Obi-Wan can do to swallow down his apprehension as Anakin ducks his head under his long tunic.
Anakin's mouth is good for more than just arguing, a wicked part of his mind whispers gleefully, and he braces his hands against the bulkhead before leaning into Anakin's body. The boy is as hot and as golden as the desert he came from, and Obi-Wan is burning alive deep in his mouth.
It's sloppy and imprecise, and Anakin is drooling down his own chin and hardly seems to care. His knees are spread open on the hard floor, and his pants are bulging between them; long like the rest of him, and just as tempting to touch, but instead of seeking relief from his own hand Anakin grabs Obi-Wan's hips and pulls him into his mouth until he's gagging, looking up at Obi-Wan with watery, red-rimmed eyes, and Obi-Wan doesn't think he’s ever seen anything more excruciatingly beautiful.
A traitorous part of his mind hisses that this is where Anakin belongs, on his knees at Obi-Wan's feet, and dark tendrils of something poisonous send pangs of lust down to his dripping cock. Anakin's name escapes his lips in a shattered groan, and his knees shudder as Anakin's teeth scrape—just this side of too rough—against sensitive skin. He reaches down and grabs a handful of Anakin's hair (too long, always too damn long) yanking his head back and staring down into the wet, insolently grinning face he helplessly paints with hot stripes moments later.
Anakin has the audacity to look surprised, almost affronted, and a mounting horror begins to creep up his spine as the weight of what he just did settles sickeningly heavy in his gut—until Anakin swipes his thumb over his chin, smearing one of the trails of Obi-Wan’s come across his skin, and pops it between his glistening red lips, watching him from beneath hooded eyes that stare him down with the single minded focus that Obi-Wan secretly and sinfully wished too often was directed at him.
He stares at Anakin, enraptured, as he does it again and again, slowly sucking his fingers clean as if he was savoring the finest of delicacies, and Obi-Wan is damned if he doesn't want to spend the rest of his filthy life feeding it to him. It's hardly moments before Anakin's face is clear, and he leans forward, taking Obi-Wan in hand delicately, almost lovingly (Obi-Wan's heart is flying in his chest, and he knows Anakin knows it) and kissing the head of his cock before tucking it carefully back inside his pants and tying up the lacings just the way they were before Obi Wan lost himself to the flames.
Anakin stands, looking down at him again, and he doesn't know what game they're playing but Obi-Wan knows he’s lost when Anakin bids him good night with another simple Master—condescending in tone but warm in affection and Force forbid, with a note of something else too—and he feels the fire roar triumphantly in his chest like a beast that can't be tamed, leaving him sated but smoldering against the cold, ungiving bulkhead.