Theory:

In the past, Dogs lived independently, just like their kin, the wolves. One day, a dog was born who wanted to serve the strongest creature on earth. He went through some animals like his kin wolf, the bear, the lion, and finally, he reached the man. He concluded that the man is the strongest of all beings and started to live with him with utmost faith and loyalty. The man became the master of the dog. That is how the dog found his master.
 
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