Kakashi's affliction
(or: how and why Kakashi started reading Icha Icha and kept at it.)
Kakashi had a strange affliction that made him different from everybody – man or woman – he had ever met. He had of course not literally asked every person he had seen in his eventful and active life whether they had the same affliction. But he had done a certain amount of scientific research when he was still very young and had just discovered his being different from the others.
He had been met with raucous laughter, enraged shrieks, confused headshakes, silent staring and even some slaps, depending on the person asked and the frankness of his question. As a kid genius, he had quickly realized the futility of doing interviews in this way and turned to conducting experiments instead. He manipulated situations that he knew would lead to those certain physical responses in both genders which he seemed to be incapable of. And the experiments showed positive results. In every. Single. Case.
Kakashi now had another reason to insist on wearing a mask over his face, since it hid his lack of physical response to those certain stimuli. He did not want to appear even stranger to his fellow Konohans than they already thought him to be.
He was at that awkward age, when everybody told you that your body would have new reactions to old situations. But his mask hid his blushes, his affliction, his sometimes gaping mouth at the view of especially appetizing décolletés, and an overly large jounin vest (which, he stated to his amused sensei, he hoped to grow into within a few years, and anyway, it did not make sense to pay for a new vest every few months with the way he was growing) hid any other "problems" situated in the groin area. Kakashi hoped that anybody who actually looked for those awkward new reactions would simply conclude that even that reaction that was just not there was simply well hidden (at that time, there was that creepy medic-nin in Konoha who wanted to write a study on the differences between puberty in ninja and civilian youths).
Then Kakashi got even better at his job than he used to be, and was kept really busy with assassinations and infiltration and protection missions, and when he had time to think about those awkward reactions again, they had already mostly gotten under control. Only that one missing reaction remained missing. He tried not to care about it too much. But still, he couldn't help but register every occurrence of that reaction in other people that happened in his vicinity (which considering his sensory awareness covered quite a large area at any one point in time). He felt a slight, irrational pang at every instance.
Somehow he got into the habit of exposing himself to the above mentioned stimuli. At first, he did it in the privacy of his home. When that showed no effect whatsoever, he decided to increase the duration of exposure. He reaped criticism for it from his colleagues, his neighbours, even absolute strangers. But as other people's opinion rated rather below his own personal health and physical status in importance, he continued his self-experiment. The criticism decreased with time, although it never died out completely. The experiment, unfortunately, showed no positive results whatsoever. At times, there was a physical reaction in his groin area. Quite often, there was an increase in blood flow in his cheeks, otherwise known as blushes. But that one missing reaction remained absent.
With time, he gave up on the idea of ever overcoming his affliction. By then he had gotten used, and maybe even a bit addicted, to the stimuli and started to expose himself to them simply out of habit and enjoyment.
He learned to fake the missing reaction: reaching up to tweak his nose when he knew he should be affected (in two instances even tweaking hard enough to cause a bleeding injury). At some points he was actually distantly glad of his affliction, for example as he heard of the defeat of the Third Hokage by the pre-genin and later student of Kakashi's, Naruto. But he never ever learned to really accept it as simply a characteristic of himself. He had spent too much time trying to be like everybody else to really accept his difference as a positive thing. He simply learned to live with it.
Nobody ever found out about his affliction as long as he lived or after, instead assuming that he showed superb control of his physical reactions. He was simply incapable of showing sexual excitement with a nosebleed. His bed partners certainly never minded.