Which animals were served by the dog? What was the reason for the dog to leave them one-by-one?

The dog was keen to have the strongest creature on the Earth to be his master who could protect him throughout his life. He stayed with a wolf, a bear and a lion before he could find his final master, human beings. But the dog was not satisfied with them and left the animals one by one because he found that the wolf was afraid of the bear, the bear was afraid of the lion, and the lion was afraid of human beings, so finally, the chose the man to be his master.