I don't think he was ever the bubbly social butterfly type. Even before the shooting of Vito he was reserved. In the GII flashback scene of Vito's birthday, Sonny even introduced him as that droopy thing, which described his demeanor and personality.

However, the cumulative effect of a series of events gradually drained his humanity. The first event wasn't so much the shooting of Vito, but the ordeal in the hospital when he moved him into another room and confronted the hit men. Obviously, he grew colder after the murder of Solozzo and McCluskey, and the deaths of Sonny and Appolonia.

It is intersting to note that upon his return from Sicily to the end of GII, Michael doesn't crack a smile. He forces a smile in Havana when meeting the politicos and is told by Fredo that a particular judge does a mean cha cha, but that's pretty much it.