When other people get more than you get, and they have done less to deserve it, how do you feel?  Jesus' parable about the vineyard labourers (Matthew 20:1-12), who were all paid the same amount even though some had hardly done any work, concluded with those who had worked all day complaining.  They had done more work than the late-comers, and yet everybody received the same wages.  Surely that was unjust.
 
But Jesus said that it was quite fair.  Those who contracted to work for a fixed reward were fairly treated because they agreed the rate for the day, and those who were hired later did not have less because they were not called to start work earlier.  Also, the landowner had the right to do whatever he wanted with the money ... his generosity was not unfair. The idea of unfairness came from the envious hearts of the day-workers, who thought they ought to get more than their agreed wage. They believed they deserved more because they had done more.
 
Jesus was teaching His trainee apostles that God’s kingdom did not operate on that basis.  Although the Jews were consumed by their duty to fulfil the law, actually, none of them obeyed it all.  But they liked to think that they were better, and that God was pleased with them, because they tried hard to be good.  That idea fuelled their pride when they thought they were succeeding.  So Jesus used this parable to point the way to His New Covenant not based on how good you are, but on grace (Ephesians 2:8-9).  It is God who has the right to say who can be accepted into His kingdom:  it is by repentance and faith in Jesus – we can contribute nothing but confession of our sin, trusting God to change our heart.
 
Many people think that being 'better' than others is good enough for God ... that they have earned their salvation.  But even those who try their utmost to be good, will fail; and if they rely on being good to be saved, they are all doomed. The generosity and fairness of the gospel is that all who repent of their failure and receive Christ's generosity will be saved, even if they have previously lived a sinful life (and everybody has by God's standard).  However, the pride and envy of legalists causes them to hate God's 'unfair' generosity to people who believe the gospel ... which helps to explain why people who try hard to keep religious rules can be so hostile to those who believe in Christ.  So, do not look for their approval: look to the Lord, especially when the hostility increases.