Why do cats have problems being introduced to new animals but not people?

[Danger Pig]

So my new roommate has a fat cat. I have a dog. When she brought the cat it did the typical cat thing. Growling, hissing, hiding. My dog just wants to go play with the cat. He is excited there is another animal in the house. The dog is twice as big as the cat. Why are cats like this with other animals?
They are like this with dogs,other cats, and other non prey animals, but not people. Why is this? The cat loves me, but still has not warmed up to the dog.
So why would a cat be okay with a perfect stranger person, but not an animal it doesn't know?

 

[Amanda ?]

Just give it time. Your cat will eventually get used to the dog in a few months.
Cats are also very territorial, so when an animal it doesn't know comes into it's space, it's going to be pissed.