Why does Scout call her father Atticus instead of dad?

This is a fantastic question. The interesting thing about
this question is that the book never really addresses why the children call their father
Atticus instead of "dad" or "father." In light of this, we can only
guess.

First, it should be noted that when Scout reflects
upon her past, she does call Atticus father. Here is an
example: 

"Jem
and I found our father satisfactory: he played with us, read to us, and treated us with
courteous
detachment." 

Second, some
might say that this practice was owing to the death of their mother. In other words,
when Jem and Scout's mother passed away, Atticus was not able to raise his children
well. Aunt Alexandra seems to suggest this. For example, she wanted to educate Scout in
proper manners. 

Third, another thought is that Atticus
wanted his children to call him Atticus. I prefer this interpretation, because there
does not seem to be a problem at all. The children call their father by the his first
name and everything seems natural. This, then, begs the question why Atticus would want
this. In my opinion, Atticus was trying to teach his children that he valued their
opinion as if they were adults.