I think that one specific theme that is shared in both works is how what one loves is critical to identity formation. Marcus defines his own identity based on the people in his life that he cares for and the people that end up capturing his moral and ethical imagination. His friendship with Will and his closeness with Ellie is representative of this. His own identity is impacted by his preferences in terms of people. This theme of how identity is constructed is a part of the novel's development. Rob's identity is defined in much the same way in High Fidelity. Rob's identity is forged through his love of music and the passion behind it. His lists to his befriending Barry and Dick are examples of how Rob's identity is defined by what he loves. Rob's passions are influential on the formation of his identity.
In both works, the ability to define oneself is not purely an internal process. It is dependent on outside forces and what one loves is where identity is created. Both Marcus and Rob construct an identity based on what they love and this theme of definition is critically in how they see themselves and how they seek to find their place in the world.