Why did Denny sell his house?

Denny sold his house primarily because he was facing financial difficulties, particularly related to his obligation to pay Mark Fein, who had been a significant financial burden due to legal fees and related expenses. Denny's situation was exacerbated by the challenges he faced after losing his wife and trying to maintain custody of his daughter, which led to mounting bills and pressure from creditors. Selling the house was a necessary step for him to manage his finances more effectively and to navigate through the challenges he was encountering in his life. This decision reflects the broader themes of sacrifice and the struggle to maintain stability amid personal turmoil that are prevalent in the story.