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.