You know how humans are really scared of snake? this fears is deeply imprinted into us, and is spread worldwide. It's what gave us dragons as one of the most universal symbols for evil, or at least for enemies.
The whole fire breathing probably derives from poisonous snakes. For example Seraphim, which are now angels, started as fire breathing winged snakes, which can be traced back to poisonous snakes. A spitting cobra with its hood can easily be turned into a winged fire breathing dragon.
My belief though is that our fear of snakes is much older than snakes themselves though. I believe this because of the sea serpent archetype.
With this I mean, it makes sense for a primitive man, or a primitive mammal that is our ancestor, to fear a snake. Stepping on one can mean death, especially in old times.
But sea snakes? they really aren't a issue for the middle east and europe, and yet we have a lot of sea serpent monsters there, like Jormugard.
My theory is that the primal fear of snake we have comes from when we lived in the sea, and we were preyed by morays/eels/oarfish or similar.
