Misunderstandings About Dog Body Language:

A Wagging Tail Is Always Happy. This is usually true, but not always.
A wagging tail, like the human smile, can mean many things. A tail that is straight up, stiff and vibrating back and forth? That is excited and stimulated, but not ‘happy’.
This dog could be moments away from aggression. A tail tucked all the way under
the dog’s body could wag at the very tip. This dog is frightened but trying. Do not press her. A tail wags level with the spine or a bit above or below spine level in large, sweeping wags? That is a happy dog!

Showing Teeth Is Always A Threat. Most of the time it is, but some dogs actually smile when they are excited to see someone. This is pretty disconcerting until you know what is going on. Breeds known for their toothy grins include Dalmations, Doberman Pinschers, Australian Shepherds, and Greyhounds. If the dog’s ears are back, the tail is wagging low, and you see a flash of front teeth, chances are you’ve just seen a grin!

Hackles Raised Means the Dog Is Aggressive. Hackles (the hair along the back) can piloerect (raise up) when a dog is excited and unsure. Sometimes it is a warning sign of aggression, but in puppies it is more often a sign of excitement, concern, or worry. Many dogs will hackle slightly (or not so slightly) when they enter a dog run, only to shake it off (literally-dogs often shake when they have calmed down a bit) in a few minutes before settling down to some serious play.

Cowering Proves He ‘Knows What He Did Wrong’. Nope, cowering ‘proves’ he knows you are about to be angry, and those are two entirely different things.
If your roommate came home once or twice a week, opened the door and attacked you for no apparent reason, you might cower or leave the room when the door opened at night, too. That would not ‘prove’ you knew what you did wrong.