Here are some possibilities:
* The Dave's are the same person: Maybe Dave was telling Alice and Harold about a time he needed help with something and enlisted another Dave to do it. The humor could come from the fact that the "other Dave" is actually Dave himself, leading to a misunderstanding.
* A silly name: The name "Dave" could be associated with something funny in their context, like a clumsy or inept person. Enlisting a "Dave" could be seen as a comical choice.
* An inside joke: Perhaps Alice and Harold know a "Dave" who is known for something strange or embarrassing, and Dave's confession is playing on that shared knowledge.
To give you a better answer, please provide more details about the situation!