Also, Michael was a hyper-controller. He had to have things his way. Sonny chose Tom as his brother, Michael didn't. Vito chose Tom as his consigliere, Michael didn't. After the Tahoe shooting, when Michael tells Tom, "You're my brother," Tom almost gets tearful and says, "I always wanted to be thought of as a brother by you, Mikie." That tells me Tom didn't believe Michael thought of him as a brother.