I'd say him growing "cold" is not exactly the essence of his nature but more like a consequence... it's due to the constant conflict within him (between his "dark side" and his conscience). Vito didn't have that conflict, that's why he is warmer as well.. not stressed out. Nothing to feel guilty about (even though he should have).
That is exactly why Mike and Vito are different, Mike has to fight himself emotionally every time when he goes closer to the mob whilst Vito was immune to it. Vito saw crime as a neccesary evil to survive in america whilst Mike knew he was doing something wrong.
However slowly he becomes engulfed in to the mafia world and becomes the mafia don we see in part 2 due to the corrupt world he becomes part of.
