Make Me a Vampire. Poof! You're a Vampire
There are two ends to Interview: the end to Louis's story, and the end of the book itself. Louis's story ends on a huge bummer. He has pretty much lost everything: Claudia, Armand, Lestat, his entire faith in humanity. He used to be able to appreciate life and art, but when he goes to the Louvre, he can appreciate neither: "I saw the people who walked there […] in a new light. Before all art had held for me the promise of a deeper understanding of the human heart. Now the human heart meant nothing. […] Like Claudia and Madeleine and myself, they could all be reduced to ashes" (3.614). Louis has nothing, which explains why he's been such a broody sad sack the entire time.
It's no surprise that Louis gets angry with the boy interviewing him when the boy requests to become a vampire, even after this Debbie Downer of a conclusion. Louis is pretty much like, "Dude, have you been paying attention to this story at all?" His story is the vampire equivalent of eating something horrible, like a rotten-egg flavored jellybean, telling your friend about it, and then having said friend ask to try it. Except with vampires, there's more eternal damnation at stake.
These two endings show us an interesting contrast. Louis seems to be telling us that eternal life is not all it's cracked up to be. (Eternal torture is more like it.) But mortals cannot understand that, and they will do anything to achieve eternal life, no matter the consequences. On top of that, it's almost as if vampirism is a symbolic form of some of nastier sex-and-violence fantasies humans have—and boy do people want to act those out.
When the boy decides to go to Lestat's estate, we have a feeling that he's going to want Lestat to transform him. Eternal damnation just doesn't have the same impact if you don't experience it firsthand.