Not One Lost

Earlier in Matthew 18, Jesus shares the parable of the lost sheep.  In it He says, "If a man owns a hundred sheep, and one of them wanders away...."  What would make a sheep wander away?  What would make a believer leave the church?  Jesus concludes the parable with "in the same way your Father in heaven is not willing that any of these little ones should be lost."  The very next thing Jesus says is, "If your brother sins...."  Is Jesus pointing out at least one cause of lost believers, their sin?  Is He also showing our part in working with the Father to keep from losing a brother?  If so, it seems His technique is counter-intuitive!  Seems more likely to drive people away!  But what if He's right?  (Scripture quotations from the NIV, 1984.)  From Matthew 18:15-20.