SONNY: No, no, no! No more! Not this time, Consigliere! No more meetin's! No more discussions! No more Sollozzo tricks! You give 'em one message—I want Solozzo. If not, it's all-out war, we go to the mattresses.
TOM HAGEN: This is business, not personal.
SONNY: They shot my father? It's business, your ass.
TOM HAGEN: Even the shooting of your father was business, not personal, Sonny!
Sonny is unable to understand that it's all really business, and none of it is personal. This is what makes him an ineffective Don and what leads to his demise, in a way. Michael, on the other hand, grasps this truth and sticks to it.