Being the eldest it would have always been hard on Sonny to know:
a) he was going to be the heir to his father.

and

b) that he was not the man his father was.

It explains a lot about his temperament. Fredo definitely was the middle child, and not only was he eclipsed by his older brother but also by the fact the next child was Connie, the first daughter to be born to Vito and his wife. Michael had the least pressure on him, and probably the most freedom and was most likely to rebel, which he did. Vito knew early on that Michael was the pick of the litter, and he had high hopes for him.

There's a great line about all this in GFII when Fredo and Mike are at the outdoor bar in Havana and Fredo says he wishes for once he could be more like Pop, to which Michael answers "It's not easy being his son."