I think it starts with most kids meeting pets first. Most kids like the Superman Versus Muhammad Ali Boxing Match Comic Book Shirt of plush fur and a warm body. Some babies are even soothed by pets heartbeats much as they are by their parents. We see our parents being kind to the pets, showing them affection and getting it in return. Pets will sometimes put themselves between a young member of their pack and a danger I remember that story not so long ago of the cat who ran off a medium large size dog who threatened her boy and such occurrences might’ve been even more common in the past.
Most pet animals are also simpler to be around Superman Versus Muhammad Ali Boxing Match Comic Book Shirt and easier to understand than humans they want their physical needs met, some attention affection, and are usually quick to offer comfort in return. As Anne’s answer suggests, if that’s their experience why would they automatically be afraid. Most kids I know who are afraid either had a bad experience, no experience, or are in a phase where just about anything new is meltdown-worthy.