Why do we say "Physician, heal thyself"?

Meaning:

Fix your own faults before pointing to the faults in others.

Background:

This proverb appears to have been in common use in the Roman Empire of 70 AD given that it is referenced as such in the Gospel of Luke, as translated in the King James Bible.

And he closed the book, and he gave it again to the minister, and sat down. And the eyes of all them that were in the synagogue were fastened on him. And he began to say unto them, This day is this scripture fulfilled in your ears.

And all bear him witness, and wondered at the gracious words which proceeded out of his mouth. And they said, Is not this Joseph's son?

And he said unto them, Ye will surely say unto me this proverb, Physician, heal thyself: whatsoever we have heard done in Capernaum, do also here in thy country.

And he said, Verily I say unto you, No prophet is accepted in his own country.

(Luke 4:20-24, King James Version).

Tradition has it that Luke was a doctor, and companion of Paul.

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