Fredo is thought of by Michael after he made a visit to his brother in the hospital, right after the Don was shot. "He remembered the middle brother (Fredo) as being physically the toughest one in the family when all of them were kids. But he had also been the most obedient son to his father. And yet everyone knew the Don had given up on this middle son as ever bieng important to the business. He was'nt quite smart enough, and failing that, not quite ruthless enough. He was too retiring a person, did not have enough force".
I think the key word here is KIDS, as an adult Fredo turned into more or less an introvert.

wil.