Aggression is a very strong word in the dog language. There are very few cases where a pooch shows any TRUE aggression. When they do however, it depends on the situation or place they are in or at....
Evolution of the species. On the humorous side: They tried being free weights, but it didn't work out. More on the development and domestication of dogs: Our dogs descended/evolved from the Grey...
Dogs are creatures that are there to keep us company. They are like a friend but a dog will (mainly) go on walks with you, play frisby (or ball) or just sit quietly and listen to you (some do bark)....