Part of it was just instinct. Part of it was the flames that seemed to broil under his skin, agitated further every time they encountered that electric aura, like lightning snapping at his fingers every time his touch came too close. A shockwave that that was pleasurable if you could hold onto it long enough; become accustomed to it; overcome the natural instinct to let go and lash back with burning fire. But it wasn't simple.
Part of it was hate. Part of it was hating someone so deeply it became something less and more than that. A favourite enemy. A frustrating acquaintance. Someone who knew and understood you too well who you saw as your opponent in everything since you had first met.
But mostly it was that when Suoh Mikoto fought with him, his fists might meet warm skin (perhaps the only sign of humanity), and that, in and amongst the returned blows from a (usually) sheathed sword, he might invade that man's space enough for those long, thin hands to grab onto his collar; to push against his chest; to shove him away. That was the kind of fleeting contact he could have. Getting him drunk never worked, and they could hardly acknowledge each other in their right minds, so if Suoh Mikoto wanted Munakata Reisi, sequestered by his cunning plans and self-righteousness as much by the enveloping blue aura, to deign to touch him, to leave the shield of his kingly powers and self-importance, there was little else to do but pick a fight.