God created the donkey and told him: "You will be a donkey!
You will work tirelessly from sunset to sundown, carrying heavy bags on your back.
You'll eat grass, you will not have much intelligence and you will live 50 years."
The donkey answered: "I'll be a donkey, but living 50 years is too much, give me only 30 years."
And God gave him 30 years.
God created the dog and told him: "You will be a dog! You will look after the man's house, you will be his best friend. You will eat whatever they give you and you will live 25 years." The dog answered: "God, living 25 years is too much, give me only 10." God gave him 10 years.
God created the monkey and told him: "You will be a monkey! You will jump from branch to branch, you will do silly things and you will live 20 years." The monkey answered: "God, living 20 years is too much, give me 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 creatures. You will dominate the world and you will live for 20 years." The man answered: "God, I'll be man, but living 20 years is not enough. Why you don't give me the 20 years that the donkey refused, the 15 years that the dog did not want and the 10 years that the monkey refused." That was what God did.
And since then, the man lives 20 years like a man, then he enters adulthood and spends 20 years like a donkey, working and carrying the load on his back. Then when his children grown up, he spends 15 years like a dog, looking after the house and eating whatever is given to him. Then he gets into retirement and spends 10 years like a monkey, doing silly things to amuse the grandchildren. That's life!