AF Hypothesis: To afford being serious, one needs to have a tremendous intelligence to compensate for a low adaptability to the environment. I am describing a character that got stuck somewhere around puberty with its misanthropy and absolutism tendencies, but kept acquiring the knowledge and experiences. (I even would put Victor Hugo into this category, to be honest.)
The basis: the inability to accept the conditions of the ambiance nor to change them, in a rigid person results in a permanent internal conflict that ultimately leads to that image of dissatisfied, a.k.a. serious, individual. With added flexibility we may see the rise in person's consciousness (yo, this sounds awakened enough. Nice!) and much more broadened horizon of norms and options that allow to view the life circumstances from the opposite or unexpected sides. Here is where humor comes into play. One of the primal principles of humor is juxtaposition. When we can perform the reasoning and debate on any subject in our head, when we can collide the viewpoints with no prejudice and make a conclusion, we cannot be serious about it anymore. It leaves our intimate space. The seriousness exists where a one-sided opinion exists. That is why serious whoever makes me want to try shaking them up or provoke a bit.
Based on the previous claims, I will dare to say that majority of the artists (including myself), film makers, and writers/novelists that I like, heavily exploit their sense of humor. It can be a very dark humor with the notes of direct mocking on a viewer, or a subtle joke inside a sentimental scene that ruins the absolute of a linearly appearing emotion. While the viewers might be going through a puzzle "what did the artist mean?!", it can be literally a play that acquired a social impact by an accident of a historic shift. (In general, it is an artificially taught principle that the viewers must sit and contemplate on what the artist meant, but this is a separate subject.)