It seems that when the Lord was making the world, he call man aside and bestowed upon him 20 years of normal sex life. Man was horrified, but the Creator refuse to budge.
Then the lord call the monkey and gave him 20 years. "But I don't need 20 years," said the monkey "10 years is plenty." Man spoke up and said "May I have the other 10 years?" The monkey agreed.
The Lord call on the lion and gave him 20 years. "Thw lion too wanted only 10 years. Again man spoke "May I have the other 10 years?""Of course," said the lion.
Then came the donkey who was also given 20 years. Like the others 10 years was enough. Man again asked for the spare 10 years and he got it.
This explains why man has 20 years of normal sex life, 10 years of monkeying around, 10 years of lion about it and 10 years making a jackass of himself.