Tax collectors and learning
In Mark 2, Jesus had just met Matthew (aka Levi). Of course Matthew had friends in his business of tax collection. These people were generally greatly disliked because they were often corrupt.
Jesus had gone for dinner to Matthew's house, and Matthew had invited a large group of his friends over. Now, the establishment religious people were still analyzing Jesus, and they really didn't understand how he could mix with this sketchy crowd.
Jesus replied to this by pointing out His purpose. He likened Himself to a doctor making house calls. He wasn't needed by the healthy people, but He could be effective with people that were sick. The sickness in this case was over value systems. He said He came to call people to repentance; righteous people didn't need that call, but sinners did.
Righteousness is the doing of God's value system. Clearly the tax collectors had failed here, so Jesus was there to open a channel of relationship between them and God. They needed a better value system; one that they could use first in relationship with God, and then also with everybody else.
The establishment people already considered themselves to be righteous. Certainly we know they weren't righteous because they didn't live out God's values. And so also they removed themselves from being able to be helped by Jesus because they didn't consider that they needed Jesus' help. I don't think they got the irony in Jesus' reply here! The point is that we remove ourselves from being able to being helped by God when we assert that we are already good enough.
The tax collectors knew they weren't "good people", although they might not have admitted it verbally and they didn't know the path of how to get there. However, Jesus came to them to show them the path. And the point for us is that Jesus comes to us, as we are - imperfect, often not ready to admit where we fail. Especially not ready to admit where we fail at righteousness - the living of love like God's. It doesn't bother Jesus that we are imperfect because He came to bring us the good news of how to repair that.
We have to admit that we need to learn and grow as God guides us. It's a state of attitude. For students during school season, there is always something to be learned and done. We are most successful in school when we have an attitude of learning and doing. The exact same thing is true of life with God and life in relationship with everybody around us. We are most successful in life when we have an attitude of learning God's values and then living them.
Of course, this is very, very hard, but God will give us help with that too!