It is, perhaps, the most popular of genres within fanfiction, fandom and beyond. It fuels fantasies, inspires scads of stories and makes our hearts ping a little. Why is it such a draw?
In thinking about the series, I recall fewer incidents than we'd probably like to have seen, and most of them do involve the Russian. The most notable, of course, comes in the Children's Day Affair, after Mother Earth (Wait, it's Mother Fear ;), lashes Illya with her strop.

Of course it is the elusiveness of that chest hidden in the shadows that compels us to keep looking, but the scene has the potential of being a very poignant moment for the partners: Napoleon wincing in sympathy at his friend's pain while Illya tries to emerge from his torment. He has, by this time, been discovered because of some very loud moaning; I'm guessing it's a tactic and not a low pain threshhold ;)
Anyway, Hurt/Comfort. Tell me more, why is it so compelling?