"All human actions have one or more of these seven causes: chance, nature, compulsion, habit, reason, passion, desire." –Aristotle
chance
The first time it happens, Reid chalks it up to chance. He and Aaron are both tense after the last case, both the last ones in the bullpen; they talk about nothing for a few moments and, before Reid knows it, Hotch's lips are forced against his own. Before Reid really knows what's happening, Hotch has him against the wall, his breath hot against the back of his neck; it's over within ten minutes, and Hotch is almost cold when he tells Reid not to tell anyone. Reid tells himself over and over again, as he drives home, that it was a chance encounter.
nature
It's Aaron's nature to go after what he wants, even if what he wants is a skinny, long-haired genius that just happens to be a subordinate. No one has to know, he tells himself as Reid winces, the small of his back pressing against his desk, and Hotch is gripping his slim hips hard enough to bruise. This is where the dark side of him comes out, where the violent nature that he's always kept hidden is displayed in brief snatches. He never sees the bruises and bite marks in the light of day.
compulsion
Reid knows that what he and Hotch do is unhealthy—he knows that he isn't the first person Hotch has done this with—this isn't the first violent, passionate affair the older man has had. But he can't stop, even though he tells Hotch that this is the last time, has to be the last time; he always comes back for more of the bruises left on his shoulders and hips, the bite marks on his shoulders, the scratches down his ribs. And even though he doesn't know it, Hotch damns himself for his need for the scratches down his back, the red marks left on his collar, his compulsion, too.
habit
Hotch knows that what he does to Reid (and he always thinks of it that way; it is never 'what they do') is unhealthy and, he thinks, probably wrong in more ways than one, but that never really stops him from going back. He can't know, can never know, that Reid feels the same way—he would never tell his older lover, but he sometimes, in that clouded moment between sleep and wake, hates this, hates him, hates himself. But even past that, he isn't willing to break this habit—and there, he's not alone.
reason
Reid tries to use his favorite standby to explain the phenomenon that is 'them': it's lust, he tells himself, it's just what happens when two stretched-too-thin young men happen to find a willing sexual companion in the other. He tries to use his favorite standby to predict Hotch: tonight he will leave just after dressing; the next, he will linger for a shower and maybe a cup of coffee or two. But one night his reason is completely undone, because, for the first time, Hotch stays.
passion
There is no denying that there is passion between the two profilers in the darkness of a dimly-lit room, but it begins to become clear that there's passion in other places, too—whether or not either one of them wants to admit it. This becomes even more obvious after their Los Angeles case, after Hotch stumbles outside, his face bleeding: Reid is the only team member outside, and he finds himself in Hotch's arms a second after seeing him. Neither knows which initiated the contact, but Hotch is gripping him fiercely, feeling him tremble against him, and their lips are pressed together in a combination of terror and need
desire
Hotch realizes, later on, that it had stopped being his body that he desired—no, he still desired that skinny frame, but it was something else. He finds himself wanting the way Reid's eyes light up when he's bringing up some new fact he's learned (because he's always learning, which Hotch never really realized before), the way he moves his lips to make that crooked, bashful half-smile; he wants him, not just in a sexual way, not in a possessive way. It takes him too long to realize it, he thinks.
"Love is composed of a single soul inhabiting two bodies." –Aristotle
