i think for some it has to do with fetish. i think others may do it to "correct" themselves. if you, for whatever reason, don't identify with your body as it is, it may become appealing to "fix" it.
i knew a guy who had his girlfriend cut his penis in half, lengthwise, bit by bit. i don't know their reasons, but they're an extremely happy couple. and you'd never suspect what they're into to see them on the street.
my worry is not the castration itself. i don't have any say or concern with what people want to do to themselves.
what worries me is the number of men who have to resort to seeing a "cutter" because the medical industry won't perform their surgeries. these people are for the most part unskilled, and many don't even work within the medical profession. harks back to the coathanger abortion days. if these guys really want it that badly i think they should be entitled to do whatever they want to themselves.
eta: the documentary show "kink" had a segment on friday about a eunuch. he told of meeting a cutter in some hotel room somewhere. long story short, the guy left him, uh, half done, and went out for a smoke. he's lucky he didn't bleed to death. i'd much rather see these guys in hospitals under sterile conditions.