Creation

God created the donkey and said to him, "You will be a donkey. You will work untiringly from sunrise to sunset carrying burdens on your back. You will eat grass, you will have no intelligence and you will live 50 years."
The donkey answered, "I will be a donkey, but to live 50 years is much. Give me only 20."
God granted his wish.

God created the dog and said to him, "You will guard the house of Man. You will be his best friend. You will eat the scraps that he gives you and you will live 30 years. You will be a dog."
The dog answered, "Sir, to live 30 years is too much. Give me only 15."
God granted his wish.

God created the monkey and said to him, "You will swing from branch to branch doing tricks. You will be amusing and you will live 20 years. You will be a monkey."
The monkey answered, "To live 20 years is too much. Give me only 10."
God granted his wish.

Finally god created man, and said to him, "You will be man, the only rational creature on the face of earth. You will use your intelligence to become master over all the animals. You will dominate the world and you will live 20 years."

Man responded, "Sir, I will be a man but to live only 20 years is very little, give me the 30 years that the donkey refused, the 15 years that the dog did not want and the 10 years the monkey refused.
God granted man's wish....

And since then...
Man lives 20 years as a man, marries and spends 30 years like a donkey, working and carrying all the burdens on his back.
Then when his children are grown... he lives 15 years like a dog, taking care of the house and eating whatever is given to him, so that when he is old, he can retire
and live 10 years like a monkey, going from house to house and from one son or daughter to another, doing tricks to amuse his grandchildren