On the 7th day,  one god said to the other.

“This one is good.  It smarter than the rest, but weaker.  That will bring balance.  How long should it live for do you think?”

“Oh I would say no more than 60-70 years maximum,” answered the other god.

“But why so short,” asked the first, “the trees live much longer than that, for example.”

“Well yes,” said the second, “but then this one’s body will wither.  It bones become brittle and life will be difficult and unpleasant  – why have it suffer?  Better to have it end and start in a new fresh body.”

To which the first agreed readily.