Dogs bite and scratch. Hitting is a human thing and some have even suggested the entire nasal sinus system is one huge punching bag for the human face. Just as we might be freaked out if an animal bites us lightly they freak out when hit. All their fur tends to protect them from bites and scratches, but not punches.
It's not the hit, it's the fact that you want to hit them.<br />
Dogs are extremely intelligent, if your angry enough to hit them, they know they have done wrong.<br />
If all you do is hit them, then they learn to hate you.
Unless it whips out a .22 and threatens to pop a fat one in yo ***, you should refrain from hitting your dog at all.