I will repeat what I have said elsewhere. Michael and Santino both resented and were a bit jealous of Tom's relationship with Vito. In many ways Vito understood that Sonny was nothing without Tom, and that if Michael was to have a "legitimate" careed, only Tom could be the intermediary.
The result was all the mean spirited things both Sonny and Michael constantly said to Tom.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
