This parable is extended because of the failure of His hearers to understand Him. "This parable spake Jesus unto them: but they understood not what things they were which he spake unto them," verse 6.
The 'you' spoken of here are the Pharisees to whom He was speaking at the end of chapter 9. We are not surprised therefore when they did not understand Him! See verses 4, 26.
He begins with, "Verily, verily" or of a truth! He is adding emphasis to the importance of His words.
In this parable the Saviour is showing the character of true shepherds and how the Pharisees do not have such a character while He does!Naturally, this does not please them, verse 19.
But it is also to be noticed that the word of Christ creates a division, 19-21. That has ever been the case!