Thoughts on Mark 2:15-17...
Matthew was elated and thankful to Jesus, as a newly converted person is apt to be. He caused a dinner to be prepared for Jesus and the missionaries. Jesus accepted the invitation, because he would come into contact with broken souls.
While he was reclining at one of the tables, publicans and shameful people crowded in and joined in the meal. The scribes didn't like this. The publicans and shameful people were low class: they'd been ousted from the synagogue. The scribes voiced their disapproval to the missionaries.
Jesus heard them. He knew that his action would be an offense to these hypocrites. And so he reminded them: The sick need a doctor.
That's true on the spiritual plane too. They that are righteous, need no doctor for their shame, since they're not conscious of it and can't be on account of its absence. Many imagine themselves to be perfect. And believing themselves to be so, they want nothing of the Savior, they won't believe that his mission concerns them.
And so Christ confines his work to the shameful. He gives them the assistance they need, he imputes to them, he gives them, his own righteousness, and thus makes them well.