These fantasies tend to get big and operatic because there isn't any reality testing--people fantasize huge, intense, heavy group scenes where they are the center of attention of some pretty tough characters....Top or bottom, but funny how few see themselves on the periphery...
When younger I used to more or less take a guy at his word and give him what he asked for, sometimes painfully sincerely. On the other hand I'd tell a guy upfront that it wasn't going to work to make him stop thinking about it --that he'd hate it but still jerk off to it afterwards, with the added 'fuck-you' that he'd damage his own fantasy of what he wanted.....meaning that he'd still want it and think about it all the time but be so afraid after too big a trip to start with that he'd only do it again when drunk or loaded. That way he'd get the worst of both worlds, a trip that was more emotionally traumatizing than physically painful and have to take years to sort it all out. Personally, I don't like causing emotional pain, just a lot of physical pain, some days more than others, and only with guys who convince me they want it.
On the other hand, why do some guys want pain at all? I think that's exactly the same kind of unanswerable question as 'why are some people gay...or straight...or bi?' They just are, and the sooner they tell the truth about it all the better their life works....but that's just my opinion, one of ....well, many doesn't cover it.....