Mike was a happy person before taking over as head of the family. But then he operated as Don without a heart. Yes, he always did "what was best for the family" and business, but still... Vito had a heart and was a great Don.

So what was it in Mike that made him so cold?

It could have been a number of things.Maybe hiding out in Sicily for so long began hardening his heart.As he missed being in America.Then there was the death of Appolonia,which i believe contributed to his change.Also in the novel it says that he went through three years of the Dons tutelage,and like you said ,although the Dons wasn't as cold,Michael had had different things and aspects of his life from Vito's while beginning his "training".Which made him a lot more colder.Just some of my thoughts. Also Vito had never lost a wife also.

Agreed... AND.... for Michael it was different. Vito became Don from scratch. Michael had the bullseye's on his back from jumpstreet.

Son of a Don
Not as good as Vito in some eyes
Saving family
Inexperienced Don


He had to bulldoze forward. Too much is made of his coldness. He was a perfectionist and it showed in his success as Don.

You can argue that a succession of traumas--his father's shooting, his murder of S&M to "protect" his father, the Sicilian exile, having to give up Kay and college, Appolonia's murder in his stead--hardened him and made him cold. We did see him playful in a deleted scene before Vito was shot.
But coldness is in one's nature. Look at the flashback scene at the end of GFII and you'll see the future, below-zero Don Corleone.

It may have to do with the fact that he was dragged into his role by circumstance as much as choice. He did not want to be a man like his father, but when his father was hot it became evident to him that unless he intervened his father and all he built would be gone. Had he not gone to the hospital that night, Vito would have been shot dead. Had he not kiled Sollozzzo and McCluskey Vito would be dead. He knew in his heart that Santino was not a good Don, and he sensed that Tom lacked the toughness to be a wartime consigliere. You can see the transformation from "nice college kid" to cold blooded killer as he sits in the chair hatching the plan to kill Sollozzo, it is a brilliant scene which ends with Michael telling Sonny "its not personal, its purely business."
As much as there is talk of a man having but one destiny, I do not believe Michael thought that was the case. Hebelieved he could be the master of his own desitny, and when that belief was chattered it transformed him.

In a way, Michael had the zeal of someone who converts from one religiion to another. Once he became a mafioso, he was going to be the meanest, toughest and coldest of them all. On top of that Michael was always something of a loner anyway. With that kind of personality, that kind of life change, and that kind of power, you have a lethal combination.