Notwithstanding the father's love, the younger son wants to see him dead, so he can receive his inheritance, while the elder son is self righteously preoccupied with the "good standing" he has "earned". Their father's unconditional love for them is completely beyond their comprehension.
The younger son begins to understand his father's love only after his rebellion bears its bitter fruit, he is desperate, helpless and humbled, and finally returns desiring just to be a servant -- yet upon his return he experiences the love of his father who prepares a kingly feast for him and restores him as a son.
What happens in the heart of the elder son is uncertain. He has been working in the field seeking to earn his father's love and acceptance. So he explodes with self-righteous anger when his father prepares a feast for his undeserving delinquent brother. Since his understanding of love is conditional, he can't believe that his father would reward his brother's behavior -- and he jealously wonders why his Dad has never lavished him this way. No hint of love or joy over his brother's repentance or his father's relief when he returns. Instead he calls him the "son of his father" (Luke 15:20), and is filled with bitterness and accusation. Perhaps it amazed him to begin to realize that his father's love had all the time been unconditional and freely given without measure simply because they are his sons. We can only hope he finally softened under that realization.