Theory:

Once upon a time dogs were their own masters they were not domesticated. They were like their kin the wolves until one day a dog was born who was not satisfied with his life. The dog was tired of wandering for food and getting scared of powerful animals. He thought about it and decided to serve the strongest creature on earth. So, he started looking for a master.

He started walking and finds his kin the wolf. The big wolf was muscular and aggressive. The wolf asked the dog where he was going? The dog said  he was looking for somebody he could work for. The dog asked the wolf if he would like to be his master. The wolf agreed immediately, and the two of them started walking together.
 
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They walked for some time. Suddenly the wolf lifted his nose and started smelling something and immediately ran deeper into the forest. The dog was amazed and asked the wolf, what has happened, and why is he running away. The wolf said that he could smell a bear and asked the dog to run as well, or the bear would eat both of them. Now the dog realized that the bear was stronger than the wolf, so he left the wolf and chose the bear as his master as he wanted to serve the strongest creature on earth.

The dog asked the bear to be his master, and the bear agreed immediately. The bear suggests they can find a herd of cows so that he will kill a cow and both of them can eat and fill their stomachs.
 
As the dog and the bear walked on towards the forest, they saw a herd of cows. But when they were near the herd of cows, they heard a loud noise. The cows were frightened and were running hither and thither. The bear saw the commotion from behind the tree and ran deeper into the forest. The bear said to the dog that he should have gone to some other place instead of coming here for the food. The lion is responsible for that place. The dog couldn't understand and asked who the lion was. The bear said that the lion is the strongest creature on earth. The dog again changed his master as he wanted to serve only the strongest animal on earth and left the bear.
 
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The dog went to the lion and requested him to be his master. The lion agreed, and they both were together for a long time. The dog was enjoying with the lion as no one messed with him, and he had no worries at all till he stayed with the lion.
 
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But one day, both of them were walking side by side on a way which was between bare cliffs. Suddenly the lion stopped got enraged and gave a big roar. He struck the ground with his paw with such a force that a hole was formed there. Then quietly, he started walking back. The dog was surprised at this gesture of the lion and asked him what the problem was? The lion said that he can smell a man around there and that they should run, or they will be in some trouble.

The dog understood that and said goodbye to the lion, as he wanted to serve the strongest creature on earth.
 
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The dog joined the man and continued with him. He served him with complete loyalty. This happened long, long ago, and till date, the dog is with the man with the utmost honesty and doesn't know any other master.