The Don is said to constantly be reminding Sonny of the foolishness of expressing anger. When talking to Michael about the utility of someone like Luca, he speaks dismissively of Luca's tendencies towards rage.
He didn't even want to see Luca privately at the wedding. But was sure to call him his "most valued friend" when they did meet briefly.
“I never lie because I don't fear anyone. You only lie when you're afraid.” ~John Gotti
