Before I start, I would like to link you to the website Curious Expeditions, which I think is very great and you might think so too.
Mal du Siecle is closely related to Weltschmertz (though the latter seems to have started a little earlier. The most famous sufferer of Weltschmertz is Goethe’s Young Werther.) Both conditions seem to generally occur in sensitive gentlemen who become very sad or passive because they do not feel at home in the current society.
“Mal du siècle, which can be roughly translated from French as “pain of the century,” is a term used to refer to the hopelessness, sadness, disillusionment, and melancholy experienced by primarily young adults of Europe’s late 19th century, when speaking in terms of the rising decadent movement. This may also simply be termed the “ennui.”” (Wikipedia)
“Mal du Siecle is a mood of melancholy and pessimism associated with the poets of the Romantic era that arose from their refusal or inability to adjust to those realities of the world that they saw as destructive of their right to subjectivity and personal freedom—a phenomenon thought to typify Romanticism.” (Encyclopedia Britannica)
The sufferers of Mal du Siecle felt that the world was now dominated by money and fame and older aspects, for example honor and beauty, were therefore lost.
Weltschmertz is related to Melancholism, of which Friedrich is a famous protagonist.