Louis contracted tuberculosis when he was six by being made to sit at the bedside of his dying older brother, the Duc de Bourgogne. It was a traumatic experience in many ways for a small boy, especially since he himself became quite ill. Louis-Auguste was generally regarded as unhealthy and not likely to live to adulthood. Several members of the French royal family, including Louis' parents and brother, had already died of consumption. Louis managed to survive with the proper care. Nevertheless, tuberculosis is a disease which can remain inactive for many years but can later recur. It can have many side effects, including depression.
The tuberculosis would come back to haunt him, infecting his baby daughter Sophie and his oldest son. I think seeing Louis-Joseph die just as he had watched his older brother die long ago revived a lot of the childhood trauma. Death from tuberculosis is not pretty to watch. I am of the opinion that since the death of his oldest son, which coincided with the beginning of the Revolution in 1789, Louis XVI was suffering from clinical depression. In the past, he had acted with much more energy and decision. This is one of the reasons Marie-Antoinette had to become more involved in the political arena during the Revolution.
I think Louis struggled with "melancholy" at various times throughout his life, perhaps due to the childhood infection with tuberculosis. Louis was a man accustomed to strenuous exercise, especially hunting and riding, not to mention his labors as a locksmith. It is my belief that he needed the fresh air and the exertion for both his mental and physical health. With the regimen of exercise and his strictly adhered to routine he was able to keep melancholy from overwhelming him. He was deprived of much of his riding after October 1789 and it had a devastating effect upon his health and state of mind. Losing two of his children, his authority, his home, seeing his people and family suffer, and being deprived of the exercise and fresh air vital to his health, left him in a very bad condition.
If we consider the courage with which Louis XVI faced the worst moments of crisis, including his death, then he is to be admired, especially in the light of everything else. The Queen is to be admired as well, for she could have slipped out of the country with her surviving children and left Louis to his doom (there were many plans for her escape) but she refused to budge from Louis' side. She would not leave him to face the disasters alone.