Jesus told the disciples he was going to be betrayed, die, and come back to life but the disciples were more concerned with who is the greatest among them. Jesus takes this opportunity to teach them about humility and being a servant. He takes a small child and puts the child on his lap and tells his disciples, be like this child and serve others. It's so easy to turn inward and be selfish, to gratify and satisfy the flesh, which is sin. Jesus takes that same child to show the dangers of our sin and causing others to sin. Then he tells them to be salt and add some flavor in the world.