Jesus is addressing people who might find themselves in this parable as the workers who had been there all day. They (we?) have done everything that was expected of them and had received what they had expected to get. Why are they resentful? What in us makes us unhappy in a situation in which we got what we had been promised?
Put yourself in the place of the latecomers. When have you gotten more than you had earned? What was your reaction? Why was Jesus silent on their reaction?
Put yourself in the place of the landowner. How should you pay your workers? Who has worked for you? Did you always reward them according to what they deserved to get? Have you ever been generous beyond what equity would call for? If so, were you confronted with more unhappy or more happy recipients?