Why does the narrator say he is divulging his secret?

From the text:

"I know that in writing the following pages I am divulging the great secret of my life, the secret which for some years I have guarded far more carefully than any of my earthly possessions; and it is a curious study to me to analyze the motives which prompt me to do it. I feel that I am led by the same impulse which forces the un-found-out criminal to take somebody into his confidence, although he knows that the act is likely, even almost certain, to lead to his undoing. I know that I am playing with fire, and I feel the thrill which accompanies that most fascinating pastime; and, back of it all, I think I find a sort of savage and diabolical desire to gather up all the little tragedies of my life, and turn them into a practical joke on society."