An adjective describing someone whose mind is unreasonably preoccupied with something, whether a person, an object, an idea, or whatever.
Someone can be obsessive about just about anything. There are people who are obsessed with patriotism, Elvis Presley, death, cheese, computers, sex, comic books, keeping clean, cats, Linux, religion, nudity, baseball, children, Terry Pratchett, boy bands, rescuing people in trouble, "Star Trek," "Star Wars," exercise, postage stamps, guns, serial killers, food, complaining, Princess Diana, music, talking constantly, talking constantly about "The Matrix" and "Fight Club," monkeys, books, Brian Eno, money, former lovers, and even themselves.
Luckily, it's not possible to be obsessive about Everything2. We're all perfectly normal.
Excuse me. I need to go check nate's webcam.