Step-by-step explanation:
An explanation according to the theory of evolution is that:
People are predisposed by evolution to fear certain things, such as snakes and the dark that would have been dangerous to our ancestors.
Hence people tend to be exposed to a lot of negative informations regarding snakes and the dark, which make them developed negative response and fear towards them. And since cats and dogs are not associated with this negative believes, people tends to preferentially associate them with positive responses.