Suoh Mikoto can be a bit slow sometimes.
He never appreciates until he has lost.
Take Tatara for instance. The puppy was always following him, all through middle school and adulthood, and nothing would stop him. Even ending up in the hospital did not deter him, and even had the audacity to make the King scratch his foot. He took that devotion seriously when he declared his wish to be the Red King's vassal, eventually working his way into HOMRA. His service was dispensable to most, since many recognised that he was not the strongest member and did not show the violent disposition that was characteristic of HOMRA members.
But Mikoto knew. He knew that Tatara did not need someone to fight his battles for him. He knew Tatara's class clown act gave the members something in common - being the member in the limelight for all the wrong reasons, but in the end everyone was drawn to him. He could have been the mascot, maybe, a mascot that cooks and cleans and laughs and initiates and makes peace. An all-in-one package matron that added a little homely touch to HOMRA.
Yet just as his life brought people together, so did his death. Tatara's passing was felt so vehemently that all of HOMRA's members were involved in the hunt for his murderer and its accomplices. Suoh Mikoto was not excluded.
On the other hand, there's also Reisi. The man that has always stood up to him, sometimes infuriatingly cool and at other times surprisingly passionate. Instead of wiping him off the slate Reisi took him into custody. Instead of forcing him to yield Reisi wanted him willing. Instead of rising to his bait Reisi actually considered the thought of 24/7 company.
Mikoto knew it was a cheap thrill, but spending forever with an equal - someone who could actually kill him - was not an idea that he disliked. Oh, he definitely could see that possibility, especially when the raven was pinning him down into the snow-covered ground and his lips were just that bit of distance away from his own. For a moment he could not figure out if the wave of heat was one of arousal or offense; he only knew that it matched the intensity of their shared gaze. So great was that wave that he maintained indifference to bring it down and extinguish the thoughts that ran with it, just so that he would have his fight and maybe prove that forever was a possibility.
Alas, Suoh Mikoto was slow - too slow in finding the murderer (that it had to be served to him on a silver platter); too slow in his fight against Munakata Reisi which only deteriorated his Sword of Damocles; too slow to prevent his Weissman deviation from hitting its limit.
But not too slow for Reisi's sword to run him through; not too slow to thank him for saving him.