Severus is obssessed with rules and discipline because it is so necessary for the young wizards to have them in order to be safe with their magic and with each other.  But I think that my Severus is much gentler and kinder than in the books.  In a way, he backed himself into a corner when he issued the ultimatum back after the Hogsmeade incident--If you get into anymore trouble, I'll take a leaf from your godfather's book and give you a spanking.  He intended that as a threat only--something he never figured he'd have to carry out, since he trusted Harry to listen to him and not get into trouble.

But Harry did get into trouble--the worst sort of trouble and therefore in the name of consistent discipline and always doing what you say, he had to punish him that way.  Also, please remember that Sev did NOT like doing that, even Harry noticed it, he had to force himself to do it, and afterwards he says to Harry in a voice that is very close to tears, "Don't EVER make me do that again."  He also thinks to himself that that was the last time he will ever punish his son that way--since Harry is too old for it and he hates doing it anyway, though of course he isn't going to tell his son that for awhile, and Harry is not afraid of his father, indeed he loves Severus.  The one he really feared was Vernon. 

And like I said before, this is characteristic of a rather strict parent like Severus, and it is not something I personally advocate.