Animals in pain or discomfort generally are more irritable and as a result are much closer to their threshold of aggression in terms of their level of tolerance.Animals can also be quite stoic. It has developed as an evolutionary advantage to not show signs of illness or weakness. Sick and weak animals are more likely to become targets of the aggression of other animals which espouse drives to dominate and take advantage of such circumstances.Animals that have suffered pains in a particular location (e.g. @ a vet hospital) or in the presence of a particular object/person/other animal, may become aggressive in anticipation of a painful event or when reintroduced to that location. Forcing a confrontation with such an animal is generally not a wise undertaking.If an otherwise non-aggressive animal becomes aggressive when approached while resting, it should be checked by a veterinarian for any underlying medical/physiologic causes.