Betrayal
Having grown up with little reference point for healthy adult relationships, Fleetwood struggles to trust people. This is the hang up in his marriage as well as his eventual undoing. As a kid, his only real role model was his father, whom Fleetwood lost right on the cusp of his maturity. He feels betrayed by his father in this and carries that grudge into his marriage. When Gifford suggests that Mary is having an affair with Kenrick, he plants a seed in the fertile soil of Fleetwood's general lack of trust in people. Immediately Fleetwood presumes guilt, unwilling to make himself vulnerable again. His lack of trust culminates in the uncovering of Gifford's deception. This is the final betrayal Fleetwood will allow. He makes things right with Mary and Kenrick, but he abandons society completely, in favor of a retreat in the mountains.
Vulnerability in Nature
Fleetwood holds a deep affinity for nature, based upon his childhood experiences running around unsupervised with nature as his only teacher. He feels comfortable and even loved when surrounded by nature. This relationship complicates not only his social life but his marriage specifically because Mary is not adapted to his wild lifestyle. Trying to make the marriage work, Fleetwood spends more of his time indoors with her, but he resents the sacrifice. After everything falls apart thanks to Gifford, Fleetwood moves to the Pyrenees mountains in France. He finds solace in the majesty and exquisite silence of the mountains. He experiences his most vulnerable moments alone in nature, a vestigial and central component of his personality.
Social Class and Inheritance
Belonging to the aristocracy by birth, Fleetwood has enjoyed the comforts of life. He owns his father's estate, having inherited it upon his dad's death. Because he encountered so few people growing up in Wales, Fleetwood fails to understand the significance of his privilege. He marries a woman of a lower class, and their distinct worldviews prove dramatically incompatible because she possesses entire types of fear which Fleetwood can't understand. He's never worried about money, theft, or abandonment in his life. When Gifford and Kenrick enter the picture, they shine more light on Fleetwood's ignorance. Gifford is there to make himself heir to his cousin's fortune, but Fleetwood doesn't even see it because he can't imagine the estate something to be stolen. This is one more piece of evidence to support his being out of touch due to his elevated social status.