Final answer:
Arthur likely experiences a mix of emotions like nostalgia and introspection as he looks through his youth's mementos, reflecting on past relationships and experiences shaping him.
Step-by-step explanation:
As Arthur sits at his desk looking through the mementos of his youth, it is plausible to infer that he is grappling with a myriad of emotions and thoughts. Such moments are reflective and often accompanied by nostalgia, a longing for the simplicity of the past, or contemplation of the uncertainty of the future.
Given the provided context of his restlessness and the way he holds onto memories such as the little tooth, it's likely that he feels a combination of excitement for the new chapter in his life, sadness at leaving his familiar world behind, and anxiety about the unknowns that await him.
Arthur's introspective moment may also involve retrospection on the relationships and experiences that have shaped him, as evidenced by the sentimental value he places on items from his past. Nostalgia and introspection are key themes in such a scenario.