Because the dog annoys the cat.
I think it might have something to do with their ancestors in the wild. Although I don't think dogs would have actually hunted cats or anything, they are natural competitors - probably hunting for the same things etc...
It's because the dog tries to go into the cat's territory. I have a book that explains it about all cat species and their behavioral patterns.
Glad to help you out
Actually dogs and cats don't hate each other, in fact in most cases if they are introduced slowly or raised together then they get along great! The only reason they fight at all is purely territorial reasons. Here is a great video to check out. Hope this helps!