Why does the dogs bark

barking is a way of communication for dogs.... many reasons behind barking... such as warning for danger,dogs bark when they play...they bark when they need attention or want some thing.

It should be:- Why do dogs bark? Don't need to use article "the" before dog because doesn't a specific dog barks, all dogs bark. And more thing, you've used dogs as a subject and as it's plural. So you should use "do" as a helping verb of it, not does.