“When Jesus heard it, he saith unto them, They that are whole have no need of the physician, but they that are sick: I came not to call the righteous, but sinners to repentance.”
On hearing this, Jesus said to them, it is not the healthy who need a doctor, but the sick. I have not come to call the righteous, but sinners — the medical analogy is both self-evident and subversive. A doctor who refuses contact with the sick in order to maintain personal health has misunderstood the doctor's vocation. Jesus is the physician of Israel, and the sick are precisely his practice. The second statement — I have not come to call the righteous, but sinners — contains an irony: the righteous may refer to those who consider themselves righteous (the Pharisees) rather than those who genuinely are. Jesus is not conceding that the Pharisees have no need; he is diagnosing the condition that makes them unavailable to the physician: they do not know they are ill.
Church Fathers on Mark 2:17
After that the Lord taught at Capernaum, He went to the sea, that He might not only set in order the life of men in towns, but also might preach the Gospel of the kingdom to those who dwelt near the sea, and might teach them to despise the restless motions of those things which pass away like the waves of the sea, and to overcome them by the firmness of faith.
Or else, after the miracle, He goes to the sea, as if wishing to be alone, but the crowd runs to Him again, that thou mightest learn, that the more thou fliest from glory, the more she herself pursues thee; but if thou followest her, she will fly from thee. The Lord passing on from thence called Matthew.
Thus then Levi, which means Appointed, followed from the custom-house of human affairs, the Word, Who says, 'He who doth not quit all that he has, cannot be My disciple.'