Final answer:
Buddy thinks Mr. Lewis is a vampire because of the common folklore and literary depiction of vampires as blood-drinking creatures that often pose as respectable society members.
Step-by-step explanation:
In the book Bud, Not Buddy, Buddy initially thinks that Mr. Lewis is a vampire due to various popular depictions of vampires in folklore and literature. Vampires are oftentimes portrayed as creatures that drink blood for sustenance and can appear as members of the bourgeois society, with respectable exteriors hiding their monstrous true natures. This idea can create a sense of mistrust and suspicion in characters like Buddy, causing them to associate unusual behaviors with the characteristics of vampires from novels like Dracula or from myths across different cultures.