*befuddled look*
Good question. Obvious answer: they're self-destructive.
From: http://www.rollingstone.com/news/printer_friendly.asp?nid=17380
Bug Chasers
The men who long to be HIV+
Carlos nonchalantly asks whether his drink was made with whole or skim milk. He takes a moment to slurp on his grande Caffe Mocha in a crowded Starbucks, and then he gets back to explaining how much he wants HIV, the virus that causes AIDS. His eyes light up as he says that the actual moment of transmission, the instant he gets HIV, will be "the most erotic thing I can imagine." He seems like a typical thirty-two-year-old man, but, in fact, he has a secret life. Carlos is chasing the bug.
"I know what the risks are, and I know that putting myself in this situation is like putting a gun to my head," he says. Some of that mountain music that's so popular is playing, making the moment even more surreal as a Southern voice sings, "Keep on the sunny side of life" behind Carlos. "But I think it turns the other guy on to know that I'm negative and that they're bringing me into the brotherhood. That gets me off, too..... Click here for rest of article