G-d created the mule, and told him, "You will be mule, working constantly from dawn to dusk, carrying heavy  loads on your back.  You will eat grass and lack intelligence.  You will live for 50 years."

The mule answered, "To live like this for 50 years is too much.  Please, give me no more than 20."

Then G-d created the dog, and told him, "You will hold vigilance over the dwellings of Man, to whom you will be his greatest companion.  You will eat his table scraps and live for 25 years."

And the dog responded, "Lord, to live 25 years as a dog like that is too much.  Please, no more than 10 years."

And it was so.

G-d then created the monkey, and told him, "You are monkey.  You shall swing from tree to tree, acting like an idiot.  You will be funny, and you shall live for 20 years."

And the monkey responded, "Lord, to live 20 years as the clown of the world is too much.  Please, Lord, give me no more than 10 years."

And it was so.

Finally, G-d created Man and told him, "You are Man, the only rational being that walks the earth.  You will use your intelligence to have mastery over the creatures of the world.  You will dominate the earth and live for 20 years."

And the man responded, "Lord, to be Man for only 20 years is too little.  Please, Lord, give me the 30 years the mule refused, the 15 years the dog refused, and the 10 years the monkey rejected."

And it was so.

And so G-d made Man to live 20 years as a man, then marry and live 30 years like a mule working and carrying heavy loads on his back.  Then, he is to have children and live 15 years as a dog, guarding his house and eating the leftovers after they empty the pantry; then, in his old age, to live 10 years as a monkey, acting like an idiot to amuse his grandchildren.