Q. Why did the good shepherd leave ninety-nine sheep to look for just ONE that was lost? Who watched over the ninety-nine sheep? Why didn't the sheep go astray after the shepherd left them?
(Submitted by: Adner)
A. You've asked some interesting questions but I think they are based on a misunderstanding of the purpose of a "parable". A parable is not intended to portray a real life situation but is a simple story told to make a point.
Here are the two slightly different versions of this parable as recorded in the gospels:
"What do you think? If a man owns a hundred sheep, and one of them wanders away, will he not leave the ninety-nine on the hills and go to look for the one that wandered off? And if he finds it, I tell you the truth, he is happier about that one sheep than about the ninety-nine that did not wander off. In the same way your Father in heaven is not willing that any of these little ones should be lost." (Matthew 18:12-14, NIV)
"Suppose one of you has a hundred sheep and loses one of them. Does he not leave the ninety-nine in the open country and go after the lost sheep until he finds it? And when he finds it, he joyfully puts it on his shoulders and goes home. Then he calls his friends and neighbors together and says, 'Rejoice with me; I have found my lost sheep.' I tell you that in the same way there will be more rejoicing in heaven over one sinner who repents than over ninety-nine righteous persons who do not need to repent." (Luke 15:4-7, NIV)
The italicized portions (Matthew 18:14; Luke 15:7) represent the lesson or the point of the parables. The shepherd and the sheep are used only as an analogy that tie human emotions and logic to the emotions and logic of God and help us better understand how He thinks.