Here's why:
* Abandoned by his father: Luke's father, Hermes, abandoned him as a child, leaving him to be raised by his mother. This abandonment left a deep wound in Luke, leading him to resent all gods for their perceived indifference to the lives of mortals.
* His mother's neglect: Luke's mother, May Castellan, was a mortal woman who struggled with her own demons. She wasn't able to provide Luke with the stability and love he needed, leading to further feelings of resentment and loneliness.
* The Prophecy: Luke's bitter feelings were further fueled by the prophecy that he would either bring about the destruction of Olympus or save it. This prophecy weighed heavily on him, making him feel like a pawn in a game he didn't understand.
* Kronos's Influence: Luke was ultimately manipulated by Kronos, the Titan who promised to give him what he craved: power and revenge against the gods. Kronos played on Luke's bitterness and resentment, ultimately twisting him into a villain.
While Luke does have a strained relationship with his father, Hermes, it's not due to any specific bitterness towards him. It's more about his general disappointment with the gods' lack of involvement in mortal affairs.
It's important to remember that Luke's story is ultimately one of tragedy. He was a young man who was abandoned, neglected, and manipulated, leading him down a path of darkness and despair. His bitterness is a reflection of his own pain and the injustices he perceived in the world.