Kakashi Hatake started out as perhaps the most mysterious character in Naruto. His signature nonchalance, calm demeanor, and his trademark face mask and tilted headband make him stand out among the other characters in the show.
His face mask had been the center of numerous rumors regarding his appearance, which is understandable since a majority of the fan community has a major crush on the coolest Sensei ever.
Kakashi was quite a polarizing character in the beginning: fans either loved him from the start, or they hated how seemingly irresponsible and ruthless he can be as a teacher. However, Kakashi soon wins hearts as he slowly starts revealing the power of "Copy Ninja Kakashi".
But he emerged as an extremely beloved character at the end of Naruto Shippuden, and for good reason. He was even praised by the Sage of Six Paths himself.
However, the only thing that bugged fans about Kakashi is his face. Fans might remember the episode from the original Naruto series where Naruto, Sasuke, and Sakura go to great lengths to try and see what is behind the mask, but in vain.
In order to put the minds of fans at ease, Masashi Kishimoto wrote a special issue of the manga where he finally revealed Kakashi's face. It was later added to the Naruto Shippuden anime as an extra: Episode 469. Needless to say, fans were not disappointed by what was behind the mask.
This was undoubtedly one of the most iconic moments of the anime, and ended the decade-long speculations in the community.
There have been many theories regarding this. One suggests that it is due to the trauma he faced as a child, with his father's suicide, that made him hide his face forever, as a coping mechanism.
Others believe that in the course of becoming the perfect Shinobi with no emotions and only a sense of duty, Kakashi went so far as to physically hide his face, and hence, suppress his emotions.
However, there is one theory that has overruled the others in terms of acceptance, perhaps because it is the whackiest of them all. According to many fans, Kakashi hides his face to hide the constant nosebleed he gets from reading the Make-Out series novels.
As fans are aware, the Make-Out novels are a series of rather lewd and sensual stories by Jiraiya. Given how excited Kakashi is about them, it is not hard to believe he would get constant nosebleeds from reading the books all the time.