Mortality
Jack is plagued with the intense fear of death. Not in the most stable mental health to begin with, he is thrown over the edge by three instances. Firstly, when he comes back from the initial visit to the cabin, Monsanto gives him a note from his mother. Jack's cat had died. For some reason, he is torn up at the thought of losing his cat and becomes disgusted with life. Next he visits his friend Baso in the hospital. Something about watching his friend dying before his eyes drives Jack insane. Once again he is reminded of how impermanent his existence is and just how final death is. Finally, after an illicit week spent with Billie, Jack takes her, Dave, and Romana back out to the cabin. Billie loves him, and he loves her. She wants to marry him, but he refuses. He says he can't allow that sort of thing. This time he completely loses it and has a mental breakdown at the cabin. When all is said and done he wakes up in the morning. As if he'd overcome his fear and made peace with death after staring it in the face, he awakens perfectly fine again.
Psychosis
Jack is incredibly psychotic. He is consumed by his dread of death and the impermanence of his life, so he desperately tries to avoid death at all costs. He has two psychotic episodes on his way to or at the cabin in Big Sur before he finally breaks down completely one night. In the attempt to avoid death, he nearly dies that night. His problem is that he's trying to control things completely beyond his sphere of control, such as life and death. Nothing he could do would change those things for him or anyone else.
Anti-establishment Sentiment
All of the members of the gang are Beatniks; they're hippies. They lead unconventional lives and look down on the rest of society for not seeing what they see. They despise the government above all else. On their way to Los Gatos, Jack and Cody have a livid discussion about the state of the America. Neither one approves of the government's involvement in private affairs. Hippies in the truest sense, they believe they should never be told what to do by any government authority figure because government is the root of all evil in the country.
Identity
Part of Jack's mental distress comes from his identity crisis. While talking to Cody on their way to Los Gatos, Jack reveals that he's conflicted within himself. He cannot figure out how to effectively communicate who he is to the people around him. Based on his appearance and lifestyle, he believes they make false assumption about who he really is. Another link to this personal crisis is Jack's relationships to the women in his life. He has this intense romantic past with Evelyn and still happily believes he will win her over one day, if only in another lifetime. On the other hand, he immediately falls for Billie and spends a wild week making love to her and drinking. Both of these women are Cody's lovers. The underlying implication is that Jack wishes he were Cody, with his sense of confidence and his success with the ladies. Doubtless this mental distress contributes to Jack's paranoid mental breakdown.
Jealousy
The Beat Generation is known for its liberated view of romantic relationships. For example, Jack, Cody, and Evelyn used to enjoy a consensual three-way relationship, but for some reason or other it ended. While Cody is traditionally married to Evelyn, he also keep his mistress Billie in San Francisco. Poor Jack knows that he has no grounds for jealousy of Cody based upon the code of his people, but he is jealous. He loves both of the women Cody loves. Worse yet, both women love him too. While Cody never lets on that he suspects or begrudges Jack any jealousy, he did break off his open relationship between his wife and his friend and intentionally left Jack alone with Billie for a week or so. Jack has so much inner turmoil surrounding his identity that it's nearly impossible for him to look at Cody without envying his care-free attitude and confidence.
