Why are people so scared of dogs?

Matan, my 10-year-old son (not pictured) was behaving like a monster the other day. He'd been in the car (literally) no more than 2 minutes and was fighting with his older sister. It bothered him that he had to share the way-back seat of our station wagon (a Peugeot 406, with back seats that open out) with her and with our stroller. It bothered him that she was talking. It bothered him that she moved her head. The fist fight was about to begin.

"Would you like to get out and walk?" (I actually said this in the calmest, most friendly tone.) Surprisingly, he said yes. We pulled over to the side and he got out. It was about halfway between his friend's house and ours. Usually he walks both ways on his own - we had only picked him up because we were already in the car. I recommended that he run home and perhaps get rid of some of that excess energy he had. We drove away and got home just in time for Nomi (our 2-1/2 month old) to start screaming.

We expected it would take Matan (aka Harry) about 15 minutes to get home. About 25 minutes later, he still wasn't home and I thought he might have taken a detour to his father's house (about 20 minutes farther than home). The social worker was going to love that story... My ex would turn it into a clear case of neglect. Um... Abuse and neglect. Why not? Then again, the social worker has gotten a bit smarter lately and would probably realize that letting a 10-year-old walk 15 minutes in daylight isn't a crime (though having only one pencil in your pencil case is a crime... that is, if your parents are divorced). Anyway, I decided to go out and see if I could spot him on his way.

As soon as I was down the steps of the building, I spotted him and quickly ran inside so that he wouldn't catch me looking for him. He came back in a good mood and sort-of-kind-of-almost apologized for his terrible behavior. Later, while we were eating dinner, he asked if we knew what had taken him so long. He went on to tell us that he was almost home, down a long flight of stairs (I think it's a full four-stories) when he saw a little dog at the bottom. The dog looked cute (that's what Matan said), but Matan is scared of dogs, so he ran all the way back up the stairs and came home the long way. Poor kid.

I wonder what makes him so scared of dogs - he's never been harmed or chased by one...