Why was McCandless motivated primarily by personal issues such as a desire to get past his troubled relationship with his family and his inclination toward thrill-seeking and adventure? Include some quotes, please.

Christopher McCandless was an idealist and an escapist. These two characteristics were symbiotic and are deeply embedded in his psyche. His idealist side affected his way of viewing the world and his relationship with it. His escapist tendencies are the manifestation of his idealism. For McCandless, the outward journey was parallel with his inward journey towards self-discovery.
A brilliant student who studied social and political issues, such as the Apartheid system in South Africa, McCandless was aware of the world's ills but felt he was one of society's victims as well. Despite growing up in an affluent household, he viewed his family life and their middle-class values as an illusion birthed from American materialism.
This is evident when McCandless wrote:

No phone. No pool. No pets. No cigarettes. Ultimate freedom... No longer to be poisoned by civilization, he flees, and walks alone upon the land to become Lost in the Wild.

He sees the human civilization as a cannibalistic one that will eventually destroy itself. He believed that by going into the wild, towards a primitive state and way of life, he could escape the poison of civilization: his parents' toxic marriage, his father's authoritarian behavior, his feelings regarding the fact that his mother was his father's mistress, and that he and his sister symbolize a family built upon lies.
These personal experiences were the micro level problems that reflected the macro level problems of the world. The true motivation for his thrill-seeking was not to experience adrenaline, but to attain a profound spiritual experience. McCandless believed that the figurative and literal wilderness is the only path towards what Buddhists and Hindus called nirvana. Like the Buddha, his outward journey was simply a means to detach himself from his family and other world worldly connections. His inward journey was his path towards enlightenment.
McCandless states, "All true meaning resides in the personal relationship to a phenomenon, what it means to you." McCandless believed that enlightenment could free him from emotional pain and childhood trauma.
https://www.researchgate.net/publication/322370084_Crossing_to_the_Imaginary_Bildungsroman_Mobility_and_Into_the_Wild