God created the donkey and told him:
you will work tireless from sun up to sun down, carrying heavy bags on your back, you'll eat grass, you will not have intelligence and you live 50 years.  You will be a DONKEY!
   
The donkey answered: I'll be a donkey, but living 50 years is too much, give me only 20 years. And God gave him 20 years.
   
God created the dog and told him:
You will look after the men house, you will be his best friend, you will eat whatever they give you and you will live 25 years. You will be a DOG!
   
The dog answered: God, living 25 years is too much, give only 10. God gave him 10 years.

God created the monkey and told him:
You will jump from branch to branch, you will do silly things, you will be amusing and you will live 20 years.
    
The monkey answered: God, living 20 years is too much, give only 10 years and God agreed.
   
Finally, God created man, and told him: You will be Man, the only rational being on this earth, you will use your intelligence to control other animals, you will dominate the world and will live for 20 years.
   
The man answered: God, I'll be man, but living 20 years is not enough, why don't you give me the 30 years that the donkey refused, the 20 years that the dog did not want and the 10 years that the monkey refused.
   
That was what God did, and since then, Man lives 20 years like a man, then he gets married and spends 30 years like a donkey, working and carrying the load on his back, then when his children leave, spend 15 years like a dog, looking after the house and eating whatever is given to him, then he gets old, retire, and spend 10 years like a monkey, jumping from house to house or from children to children, doing silly things to amuse the grandchildren.