In the Roman Empire, tax collectors weren’t paid very much, and it was generally expected by the authorities that they would line their pockets by inflating the taxes owed by the people. Rome didn’t much care as long as they were paid. But because everyone knew what was happening, tax collectors were even less popular then than they are today. In Judea, it was even worse because they were seen as traitors, helping the conquering invaders over their own people.
So Jesus comes up and turns one into a disciple. We don’t get any backstory, or why this Levi is willing to drop everything to follow Jesus, but he does and is so overjoyed that he holds a feast for Jesus. He invites everyone he knows, who are mostly other tax collectors because no one else would associate with them.
The Pharisees see what’s going on. They are watching Jesus to see what He does and says, and they can’t believe that a teacher of His standing would associate with sinners like Levi and his friends. The Pharisees tried very hard to live holy lives, to such an extent that even being near someone not following the law and traditions could damage their standing before God, as they saw it.
With His actions, Jesus refutes them completely. When they ask Him why, He responds by saying that sinners are the kind of people He has come to minister to in the first place. Righteous people don’t need a Savior; only sinners. What the Pharisees didn’t realize yet was that they were just as sinful as those they looked down on.
Do not let our sins seem less than those of others, just because they are different sins.