Da Ponte was profoundly cynical. You can read that in Cosi Fan Tutte, which is the only play he wrote for Mozart on a subject of his own. He was also quite a womanizer. I have an idea that Don Giovanni is something like a personal nightmare - certainly, the character of the Don is unpleasantly true to life, more than one would expect in an eighteenth-century drama. The knowledge of the psychology of addiction, once you look at it with care, is scary.