In retrospect, Touma comes to realize that he drifts to him because he's safer. They were all teenagers when they saved the world, but Touma was stuck being youngest and feeling oldest, and on top of it all, being confused by the whole concept of having friends. All he knew was the fierce ache in his heart for his companions, and with time he knows he saw the same thing behind Seiji's shuttered gaze.
Not many people would call Date Seiji "safer". Seiji races fast cars, carries large swords, is as perfectly clipped and polite as he is deadly. There is a concept in many languages of a final cut – a blow that is finished properly. That's Seiji. Except Touma knows that Seiji does not like to battle while desperate; that he feels the weight of his heavy sword; that he races to feel alive, in control, as electric and vibrant as the armor he's supposed to carry. Touma will probably never understand "supposed to", and likely never will. He can observe it in Seiji, though. Soon he will come to feel it.
Touma likes history because he can put quotation marks around the concepts. He can trace the genealogical footsteps of a farmboy conscripted into nation-state war and the sickle in his hands. The mythological tropes are objects he can layer into footnotes, to show Nasuti, to consult in journals. Even as he comes face to face with a halfbreed Naga, or giant spider webs, or demons who leap at you from the moon. These are targets at which he may aim. Eventually he realizes that there is no book that can contain the horror of sacred deer slaughtered and then brought back to an un-life, or the helpless feeling of life draining away even as his friends suffer. He and Seiji speak of these things.
He and Seiji speak of these things together, whispering secrets into the hollow of his collarbone where the hot blood visibly pulses through porcelain skin. Seiji does laugh at him for that one; he's used to people marveling at his beauty. Touma qualifies the statement as only being true under a full moon. At which point Seiji shuts him up, kisses him fiercely, desperately, murmuring that he's tired of talking about the night.
Except Touma loves the night. There are no temperature gradients to throw off his arrow's flight, nor bend the atmosphere away from his view of the stars. It's truly beautiful in the winter, when everyone else is inside and enjoying cozy sweaters and hot cocoa. Touma sits on the cold slate roof and tries not to associate the woodsmoke with anything other than a cheerful hearth-fire. He's not sure if Seiji leaves him be in these moments because he's not supposed to disturb Touma, or he's not supposed to climb on other people's roofs. Touma daydreams all night and all day of taking Seiji to meet the stars, to see what he thinks of their particular light.
Seiji does travel like a particle of light, straight straight straight lines along the curve of the universe. Touma knows he is also a wave. Energy. He knows it in the way Seiji grabs him in the half-dark, makes him twist and then brings him back down, softly, as though Seiji has never loved anyone like this before and cannot bear to tell him in words. It's all right; Touma understands the concept. Seiji teaches him all the things Date Seiji is not supposed to be. Touma is often seized by the urge to kiss him all over, from toes to the crown of his head, no matter that Seiji ends up laughing, drawing his callused hands along Touma's ribs and digging into his hair. No matter that Touma always forgets the map and has to start all over again.
Touma looks up and finds that they are a concept: a one-ness. "Singularity" seems too trite, and too imprecise. Touma looks around and finds pieces of their history in his room, on his walls, in the violent flutter of his chest. Touma looks at Seiji, and knows he will never forget him.