The central theme of "The Duke And I" is the complex interplay between love and deception. Simon and Daphne's initial arrangement is based on a lie, but as they navigate their fake engagement, they discover genuine feelings for each other. This theme explores how love can emerge from unexpected circumstances and the importance of honesty in relationships.

The novel delves into the pressures and expectations placed on individuals by society, particularly in the context of marriage and social status. Both Simon and Daphne face societal pressures in different ways, and their journey highlights the struggle to balance personal desires with societal obligations.

Throughout the story, both Simon and Daphne undergo significant personal growth. Simon grapples with his troubled past and the emotional scars left by his father's rejection, while Daphne learns to assert her own desires and make choices that lead to her happiness. Their individual journeys of healing and self-discovery are central to the narrative, making their eventual union all the more satisfying.