That hope reflects Vito's own evolution - when Santino was a baby, Vito was unemployed (thanks to Don Fanucci), broke, with little optimism. Vito's life was focused on providing for his family on a hand-to-mouth basis.

Compare that to a few years later when Michael was a baby - Vito had killed Don Fanucci, taken over and gained respect of the neighborhood. Then shortly thereafter, he goes to Sicily and avenges his parents. He's far more accomplished with even greater things in store. It's as if he can afford to think ahead and dream great things for his youngest son.

I'm speculating, but Santino probably experienced a greater level of poverty and uncertainty in his early years, whereas Michael spent his early years more comfortably. That would be a source of contention between the brothers.