Rich Young Ruler
A rich young man approaches Jesus, asking about  the key to eternal life, a topic that Jesus had occasionally spoken about (see this encounter , for example). But in response to the question, Jesus simply presents the law as commonly understood: five of the ten commandments. As though he expected such an answer, the young man calmly claims mastery over them. But then Jesus drops the surprise: this man needs to do more than simply follow the commandments in order to achieve eternal life. But the specific lack has the young man leaving disheartened: 

"Sell everything you have and give to the poor.... Then come, follow me."

Jesus calls everyone to follow him, but in so calling this young man, Jesus was aware of the one thing holding this man back: his attachment to wealth. Knowing that this is a common difficulty, Jesus calls it out: it is so very hard for the rich to enter the kingdom of God! Those who are wealthy tend to be all in for their own personal kingdom-building. But the kingdom of God can only have one king, and it isn't us.

For those listening to Jesus, to suggest that it is difficult for the rich to enter the kingdom of God made no sense: they had been brought up to believe that riches were God's reward for upright behavior. "Who then can be saved?" they ask.

Luke 18:18 (ESV) A certain ruler asked him, “Good teacher, what must I do to inherit eternal life?”
19 “Why do you call me good?” Jesus answered. “No one is good—except God alone. 20 You know the commandments: ‘You shall not commit adultery, you shall not murder, you shall not steal, you shall not give false testimony, honor your father and mother.’”
21 “All these I have kept since I was a boy,” he said.
22 When Jesus heard this, he said to him, “You still lack one thing. Sell everything you have and give to the poor, and you will have treasure in heaven . Then come, follow me .”
23 When he heard this, he became very sad, because he was very wealthy. 24 Jesus looked at him and said, “How hard it is for the rich to enter the kingdom of God! 25 Indeed, it is easier for a camel to go through the eye of a needle than for someone who is rich to enter the kingdom of God.”
26 Those who heard this asked, “Who then can be saved?”
27 Jesus replied, “What is impossible with man is possible with God.”
28 Peter said to him, “We have left all we had to follow you!”
29 “Truly I tell you,” Jesus said to them, “no one who has left home or wife or brothers or sisters or parents or children for the sake of the kingdom of God 30 will fail to receive many times as much in this age, and in the age to come eternal life.”
This reaction is just as common today, of course: we seem to imagine that greater wealth is the solution to whatever difficulties we may be experiencing. In contrast, Jesus tells us that greater wealth may, in fact, keep us from the real solution.