The lost sons in today's parable demonstrate a principle of sin. The youngest son wandered far from his father, squandered his inheritance, lost his identity, faith, and dignity. He recognizes his distance, and that he does not deserve to be a son, but desires to be a servant of his loving father. The older son, while he did not wander far from his father, had his heart far from the father. He simply did not even recognized his father's love. Both of these sons are in a land of dissimilarity, as St. Augustine would call it. The younger son returns from his life of sin, but the older dutiful son is left with the decision.
Those responding to a vocation need to know the love of the Father, to have their hearts with Him, experiencing His love. Whether we were great sinners who wandered far from the Lord, or dutiful servants, the Father loves us and claims us as sons and daughters.