There are many things a dog cannot resist doing. If he is planning on biting someone, he has to focus on his target, and he has to bare his teeth. If he is going to protect himself, he will tuck his ears back and his tail in a downward position and turn aside. During the dark unrecorded haze of the history of wolves, those who had the wits to recognize these things had an advantage over their more dull-witted pack-mates. Being on the lookout for the fangs or the intent stare of a stronger member of the pack was a way to keep away from unnecessary physical harm from a wolf one had no plan of testing anyway. So, being on the lookout for the averted gaze of a wayward member was a way to keep away from the needless trouble and danger of fighting with a wolf that was ready to give way without a fight anyhow.
When wolves were on the lookout for unintentionally dropped hints, it became possible to start dropping them deliberately. A wolf that can correctly read a fang or a stare as a threat can keep away from a fight and a wolf that can show a fang or fix a stare can then communicate a threat without fighting. This evolutionary advice loop between receivers and senders is what was almost behind the development and rituals of the visual signals that wolves and dogs use.
Many of these signals are related to the serious wolf business of control and submission within the pack. Dominance and threatening signals include showing the teeth, pricking the ears and gazing. Submissive and non-threatening signals include laying the ears back, averting the gaze, approaching at an angle instead of head on, tucking the tail securely under his belly and rolling over and lying belly-up which is considered a gesture of passive surrender to superior force. These signals will eventually become routine. When ever a wolf lifts his lips and shows his fangs, it doesn’t mean that he is going to bite someone; instead, this is merely a symbol of threatening intentions. At this moment in the evolutionary history of the wolf, read as such by other wolves. Wolves are inclined to read it that way because of the incontestable fact of evolutionary history that fangs really do bite people. So, wolves became used to showing fangs as a threatening sign because wolves were prone to react to fangs as a threat.
The majority of vertebrate animals acquired an instinctive admiration of another biological fact that is often exploited in visual communication. For example, huge things out there are considered more dangerous than little things. Therefore threatening or dominance-asserting wolves try to appear big. Wolves stand upright, often astride the animal they are attempting to amaze; they raise their tails and harden their hackles.
Submissive or terrified dogs try to look small by crouching low, often dragging themselves along the ground. It is essential to understand that this does not mean that the huge-looking wolf is aware of how big he appears, nor is any other wolf fooled into believing he really is huge. Once again, these are just rituals. However, they come from the fact that wolves have been created to react in ways that make these rituals operative.