Brooding is a characteristic of any emotional vampire (dark trench coat optional) in Interview with the Vampire, and the reason for all this moping seems to be one thing: dissatisfaction. Vampires have eternal life, gorgeous beauty, and the ability to travel all over the world, yet they still find something lacking in their long, long lives.
Maybe humans are prone to dissatisfaction in general. We'd love to see what would happen if a naturally optimistic creature were granted eternal life. Vampire Cock-a-Poos, anyone?
Questions About Dissatisfaction
What is Louis so broody about?
Why is Louis disappointed about having Lestat as a master?
Is Claudia disappointed in either of her "parents"?
What is the source of Armand's dissatisfaction with Louis, ultimately causing him to leave New Orleans?
Chew on This
Louis wouldn't have been as dissatisfied as he is if Lestat had just come clean with him. If they had gone off searching for knowledge together, Louis would have had something to focus on other than his dejected bitterness.
Claudia would have been a relatively happy vampire if she weren't trapped in a five-year-old's body. She'd tell Peter Pan to shove it. Never growing up stinks.