Why do dogs in parked cars tend to growl and bare their teeth?

Dogs are animals with very keen sense of territoriality, therefore, they turn aggressive when its boundaries are threatened by strangers.

Since little we are taught not to get near strange dogs because they could attack us without any previous warning. I mean, it´s really frightening when dogs growl and bare their teeth at us. Now just imagine how it must feel to be bitten by a dog.

They act this way because they also feel threatened and scared, and the best way they have to defend their territory and themselves is making the first move by attacking or at least threatening you.

That is what happens when they are kept shut in a car, they feel nervous, scared and are in permanent alert awaiting for anyone who dares trespass its territory. So don't get near dogs you don't know.