Do you think that shortly after Sheldon’s first child is born and he realizes that he can’t ask his father questions about how to be a dad and he’s crying while he’s up late with the baby, he calls Howard?
It’s not because Howard is such a great dad—though Howard isn’t at all a bad dad— but rather because Howard gets it in a way none of Sheldon’s other friends do. Howard has to be a dad without having one to show him how. Plus he lost a parent too.
And maybe the two men bond over that.