Like, in 1940s scenes before Sonny is killed, Michael just seems a step removed from his brothers emotionally, not even just from the Mafia business.
He deliberately distanced himself from his family because he wanted no part of their business. He showed some real warmth--playfulness--with Kay in a deleted scene in a NYC hotel room just before Vito was shot. He opened himself up enough to be hit with the thunderbolt when he saw Apollonia. But, after that, he was cold as ice.
