why would someone hit their kids?

I remember one psychologist I was seeing telling me many years ago, after I told her about my father's abuse, that I could break free by breaking the cycle. This was long before [Older Daughter] was born. I heard it many times before and many times since. Several people have commented on this blog and emailed me commending me for not continuing the cycle of abuse. Although I appreciate the kind words, I never understood this. It would be like rewarding my kids for NOT setting the house on fire, or commending a husband for NOT cheating on his wife. It doesn't make sense to me. Why would someone get praise for NOT doing something that they shouldn't? 

 When I finally stood up to my father and accused him of beating us, he said that he was only human and couldn't control his emotions. I told him that being human means you CAN control your emotions (to which he yelled at me some more). I've gotten mad at my kids. I get really frustrated with [Older Daughter’s] spaciness and and slowpokiness and forgetting what she's supposed to do despite being reminded of it a thousand times. And I can't even get started on [Son]. He makes me FURIOUS. (I've come to learn that he is no more horrible than other boys his age - throwing things and throwing tantrums and constantly getting into trouble is just something that healthy, normal boys do.) I've gotten upset with them, and I've even raised my voice at them, but I've never once even CONSIDERED hitting them. I've never had to hold back from hitting them, because I've never felt the urge or the need. In fact, I don't see how anyone could hit their children in anger. I don't have a problem with other people spanking their children as a form of discipline, although I would not do it myself. I know many moms who thought they never would spank, but now do. I think if it's a planned spank on the butt, and not done in anger or to take out frustrations, it may be needed. The reason I never have and never would is that my father always used to wish me the worst with kids. He used to say "I hope your children treat you like you do me," and "You'll see when you have your own kids." So even if I felt tempted to give a controlled spanking, I never would, just because I want to be able to say that I never laid a hand on my kids. But it doesn't matter, because I just could never even BRING myself to do it. I don 't know how anyone can strike a child in anger.