When the younger son returned, it was the father who got his life back on track. The father gave the son his righteousness and therefore they had a celebration. People who have always been living with their own righteousness and see those who have lived a bad life come to God and flourish get angry. How do they get those blessings when they do not deserve it? There is jealousy that is stirred in the elder brother when his younger brother is blessed instead of punished.
It is at this point where many get bitter because they do not have an understanding of God’s grace. It is when we experience Gods power in our life even though we do not deserve it, it is our God’s grace for us. For God, the younger and elder sons are both loved, Jesus died for both brothers and they both have the same grace. God, who loves, wants to pour this grace, He wants everyone to come home and celebrate.
The elder brother becomes self-righteous, and condemns his younger brother; God does not look at us from our level but His level. From Gods perspective we have all sinned and therefore we all need grace, if we do not get this grace we are all destined for hell. When the elder brother compares himself to his younger brother, he sees himself better. However, when looking from Gods level they both need mercy and grace.
It is when people forget that the Father is gracious, that they get resentful and bitter.