The owner of the vineyard is a businessman. He needs workers for his vineyard. He hires them at a set wage. Some come to work early. Some come to work late. But at the end of the day, they all get paid the same amount.
Those who have worked all day object. They want a say in what the owner of the vineyard does with his money. By their attitudes, they perceive they too own the vineyard.
This same attitude will be repeated as Jesus heals people at the temple: his critics will object to the time and place of his healings. It will be repeated again in the Parable of the Tenants (21:33ff). The workers believe that because they work the field, the field belongs to them.
Two lessons come to mind. First, those who hold power (or money) are not obligated to those who do not to share it with them. The powerless and the penniless have no right to demand a share, and no right to a share.
My guess is a lot of my Christian readers will object to this; but notice that I am not saying the “haves” have no obligation to share with the “have nots.” The obligation, however, comes not from the powerless and penniless, but from God who cares for others, who cares for the disadvantaged, and who calls all to imitate him. There is an ethic here that must be adhered to, and failure to do leads to a sense of entitlement, as well as to miss the second more important lesson. God holds all blessings. We have no right to them. Whatever He gives is a benefit we have no claim to, and no right to expect. Once we learn the first lesson, the second will come more naturally.