Luke 15:3 And he spake this parable unto them, saying,
Luke 15:4 What man of you, having an hundred sheep, if he lose one of them, doth not leave the ninety and nine in the wilderness, and go after that which is lost, until he find it?
These sheep belonged to the owner. One out of one hundred was lost. One is lost in the wilderness of the world, confused, alone and in danger. The owner leaves the flock to find the lost one. The flock could be the church gathered together and one is missing. The Good Shepherd goes to the missing one to bring him back. He cares for his lost sheep and he is concerned for him.
Luke 15:5 And when he hath found it, he layeth it on his shoulders, rejoicing.
Those who are gone astray from the Lord, he seeks, he bears you upon his shoulders with all your burdens. Is he unhappy to do this? No, he is rejoicing.
Luke 15:6 And when he cometh home, he calleth together his friends and neighbours, saying unto them, Rejoice with me; for I have found my sheep which was lost.
When a lost sheep is found and is returned to fellowship all who belong to the master should rejoice.
Rom 12:15 Rejoice with them that do rejoice, and weep with them that weep.
Rom 12:16 Be of the same mind one toward another. Mind not high things, but condescend to men of low estate. Be not wise in your own conceits.
Luke 15:7 I say unto you, that likewise joy shall be in heaven over one sinner that repenteth, more than over ninety and nine just persons, which need no repentance.
The angels rejoice as well, for that which was lost has been found.