The man, the donkey, and the dog
On the last day of creation, God was parceling out years. To man he gave 60, adding that there would be both good and bad years, but the good years would outnumber the bad years. The man, always desiring more, thought that he was being short-changed, but he held his tongue. God gave the donkey 30 years, but warned that the bad years would outnumber the good - so the donkey asked that he be relieved of 10 of those years, since they were going to be so hard. The man asked for, and got, the 10 years that the donkey gave up. Likewise the dog was offered 20 years, more bad than good, and the dog said he would be happy with 10 - and the man begged for the 10 years that the dog didn't want, and received those as well.
And that is why man lives the life of a donkey from 60 to 70, and why he lives the life of a lowly dog from his 70th year on.