From the Oxford Dictionary:
early 17th century (in the sense ‘an attack’): from Latin aggressio(n-), from aggredi ‘to attack’, fromad- ‘towards’ + gradi ‘proceed, walk’.
“Human ethologists argue that the human family represents one of the most peaceful associations of individuals in the animal kingdom. This seems to be an evolutionary trend, because humans also show markedly reduced aggressive behavior towards other group mates in comparison to our (living) primate ancestors. Many assume that this change also enhanced our possibilities for forming complex alliances, and engaging in sophisticated collaborative activities. This means that humans are very sensitive to any kind of aggression which could seriously disrupt group activities.
On this basis we can assume that during the domestication of dogs humans ensured that the animals displayed similarly peaceful attitudes, and dogs probably underwent selection for reduced aggression towards human companions (chapter citation). Thus it is not surprising that aggressive behavior by dogs has a strong influence on the human-animal relationship, and is the leading complaint in dog-owning families (Riegger and Guntzelman 1990).
Dog aggression is also seen as potentially dangerous because the patterns of human and dog behavior are not fully compatible; that is, there is only limited overlap between the two species-specific sets of behavioral signals and action patterns that cause physical injuries and pain. Humans (especially children) may have innate tendencies for judging the ‘meaning’of growling or persistent gazing, but they may not understand the signal indicated by erect tails and ears. Biting is only the last resort when it comes to aggressive interaction among humans, who prefer to use hitting as a form of physical deterrent. In contrast, the hitting element is missing from the repertoire of most dogs, but biting occurs relative often. In addition the mostly (or originally) thick fur of dogs provides some protection against the effects of a bite which can cause unexpectedly dangerous injuries in furless humans. The behavior of dogs could also vary depending on whether they perceive the situation as being social or predatory. Predatory behavior is not signaled and is aimed at destroying the opponent, so such attacks could be even more serious. (Strictly speaking, predatory behavior should not be categorized as aggression.)
With regard to aggression, the human-dog relationship is based on ‘unconditional trust’ (just like the human-human relationship). However, if this trust is lost for any reason, the original relationship will be difficult to reinstate. ” – Adam Miklosi, dog behavior, evolution, and cognition