Why does Sofie bite cats on the head?

If you say because she's a dog, you are wrong. Buddy didn't do that, neither did Meiko, and Tonka doesn't. I don't think Chibi even cared about cats.

Don't remember Meiko caring much about cats either.

Buddy did chase one of the cats we had once, he pinned her down apparently. I don't remember, and probably didn't see it happen. If the cat doesn't run, dogs are usually confused. Spike didn't like dogs, so ran from them.