First God created the ox, and He said to the ox, "Today I have created you! As a beast of burden, this is what you will do. You will go to the field with the farmer all day long. You will work all day under the sun, pulling the plow or hauling wagons! And to do this, I will give you a life span of 50 years."
The ox objected. "What, you want me to live that kind of life for 50 years? Let me have 20 years, that'll be plenty of work, and I'll give 30 years back to you." So God agreed.
Then God created the dog, and He said to the dog, "Okay, dog. What you're going to do is sit all day at the door of your house, and guard the door. Any people that walk by, or try to come in, you bark at them. I'll give you a lifespan of 20 years for this!"
And the dog objected. "What? All day long I have to sit by the door? No way! Just give me ten years. I'll give you back the other ten!" So God agreed.
Then God created the monkey, and He said to the monkey, "A monkey has to be entertaining. You get to make everybody laugh and do tricks. And I'll give you 30 years of life to do this."
And the monkey objected, too. "What, Make people laugh? Monkey tricks? Twenty years will do for that, and the other 10 years, I give back to you." So God agreed.
Then God created Man. He said to the Man, "Your job is to sleep, eat, and play. You will enjoy many things in your life. You're my special creation, and all you have to do is be good and enjoy your life. For this kind of life, I'll give you a 20 years lifespan."
The man objected, too. "What, God? Such a good life? Eat, play, sleep, do nothing? Enjoy the best and you expect me to live for only 20 years? Why don't we make a deal? Since the cow gave you back 30 years, the dog gave you back 10 years and the monkey gave you back 10 years, I will take those years! That will make my lifespan 70 years, right?" So God agreed.
And that's why...
For our first 20 years, we eat, sleep, play, enjoy life and do nothing much. For the next 30 years, we work all day long. For the next 10 years, we entertain our grandchildren by making monkey faces and doing tricks. And for the last 10 years, we stay at home, sit in front of the door and bark at people!