Motives are so subtle that we often don't even recognize our own motives. Those who are followers of Christ must be continually examining their hearts which can only truly be done through the scrutiny of the Holy Spirit. Unless I am continually spending time with God and allowing His Spirit to permeate my mind and heart, I will forever be duped by my own motives.
Peter listened to Jesus' conversation with a rich young man who was seeking eternal life. While on the surface this young man appeared to be devout and honestly seeking God, he was unaware that God was not his primary pursuit. It became apparent in his conversation with Jesus that his possessions were more important to him than anything else. Jesus told him to "sell your belongings and give to the poor, and you will have treasure in heaven. Then come, follow Me." (Matthew 19:21) But the man went away sad and Jesus pointed out to His disciples the difficulty of those who are rich being able to place God first in their lives. This prompted Peter to ask, "Look, we have left everything and followed You. So what will there be for us?" (Matt 19:27) This question throws doubt on whether Peter's motive at this point were pure. Was he following Jesus merely for the reward?
This occasion sets the scene for Matthew chapter 20 in which Jesus tells the parable of the Vineyard Workers. In this parable the land owner hired men to work in his vineyard at various times throughout the day. Some were hired early in the morning others at mid-morning, others at noon, and still others just an hour before quitting time. But the owner paid all of the workers the same amount for their labor. It is not surprising that the workers who worked all day were unhappy with this arrangement. But Jesus concluded the parable with a statement He also made at the conclusion of the conversation with the rich man, "So the last will be first, and the first last." (Matt 20:16)
Jesus was not saying that God arbitrarily reverses the order of things. That those who show up first will automatically be sent to the end of the line, so to speak. It was a statement of motive. Notice in the parable that those who went to work first, started work only after they had negotiated for their pay. But those who were sent into the vineyard later in the day were simply told they would be paid whatever was 'right,' and they accepted. Peter's question suggested he thought the disciples should have a special reward because they had left everything to follow Jesus.
Jesus was emphasizing that this is not the way it works in the kingdom of heaven. It is not about great reward for great effort. It is those who serve out of great love and no expectation who are most likely to have the greatest rewards.