...is that people who are obsessed don't seem capable of recognizing that they're obsessed.
Their own behavior goes completely over their heads, and they fail to see what almost everyone watching them sees...that they're caught up in an obsession. As an example, consider some stuck-in-his-head mental midget who gets a wild-hair-up-his-ass idea that God (yes, the same God who is on record as having smitten millions of innocent people because he was pissed off at them) became so affronted by the bombing of an obscure Christian monastery during WW2 that He withdrew His presence from the very Creation He had created, and basically told all its sentient creatures to fuck off, because He would never have anything to do with them again. OK, this is ludicrous, right? You would think that the first time said mental midget dared to say this out loud or in print he'd catch a clue from the way people reacted and realize that he was talking crazy, right? But nooooooo. Said mental midget goes on and on and on and on, trying to make other people agree with this insanity. In other words, he was obsessed with the weird stuff in his head, just because it was in *his* head. And, he was incapable of even realizing that he *was* obsessed. Crazy, right? Now imagine a lonely, 70-plus-year-old spinster who listens to this crap and not only doesn't challenge it, but develops an obsession of her own with the mental midget in question. Never having met him, she makes almost 1700 posts to or about him to an obscure Internet group that no one cares about, turning "defending him" and "explaining what he really meant" into something that similarly goes on and on and on and on. This crush on the mental midget in question gets so bad that she embarrasses herself thoroughly, resorting to lies and insults to "defend" him, and destroying whatever little credibility she had on the forum. And throughout all of this, it doesn't ever occur to her that she's obsessed. She would probably bristle at the idea and write dozens of stinging replies pointing out how absurd the idea is. Come to think of it, she probably will...