John George IV of Saxony was lovesick

As a child Caroline of Ansbach (1683-1737), George II of Great-Britains’s Queen Consort, had to cope with a hostile stepfather. In 1692, after the death of her father Johann Friedrich of Brandenburg-Ansbach (1654-86), her mother Eleonore Erdmuthe of Saxe-Eisenach (1662-96) married the 6-year-younger Elector John George IV of Saxony (1668-94). He had a preference for his teenage mistress, Sibylle von Neitschütz (1675-94), a daughter of his late father's notorious mistress. His father had tried to keep both his sons away from Sibylle, because she may have been John George’s illegitimate half-sister.

While Eleonore suffered from miscarriages and phantom pregnancies, Sibylle gave birth to a healthy baby girl. With her hold over the Elector increasing, Sibylle continually worked upon John George’s distaste for his wife. 

John George had had violent tantrums since his youth. Once a marital quarrel grew so fiery, that the incensed John George wounded several persons, and would have stabbed Eleonore, had he not been disarmed by his strong brother August, who tore “three blades, one after another, from his brother’s hand”, while “cutting all his fingers”. Later, John George forcefully bedded his hated wife again. Eleonore was now constantly exposed to “hard usuage”, and even feared for her life. 

Suddenly, Sibylle became ill and lay sick for 9 days. She was diagnosed with smallpox. All the time, John George remained at the bedside of his beloved mistress. Five doctors were consulted, but to no avail. Sybille died around 7 a.m. on April 4, 1694. She was only 19 years old.

Due to John George’s immoderate grief “a black melancholy and distraction were visible in his looks and behaviour”. In the night of April 19, little spots appeared on his body, too. On April 27, John George grew light-headed, and “continued raving mad”, until he expired at 5 p.m. at the age of 25. 

See my full biography: Elector John George IV of Saxony (1668-1694).

Another theory about British King George III's illness

Modern medicine may help us to discover the real reasons behind King George III's erratic behaviour, writes historian Lucy Worsley. Using the evidence of thousands of George III's own handwritten letters, Dr. Peter Garrard and Dr. Vassiliki Rentoumi have been analysing his use of language. George III, when ill, often repeated himself, and at the same time his vocabulary became much more complex, creative and colorful. 

These are features that can be seen today in the writing and speech of patients experiencing the manic phase of psychiatric illnesses such as bipolar disorder. Mania, or harmful euphoria, is at one end of a spectrum of mood disorders, with sadness, or depression, at the other. George's being in a manic state would also match contemporary descriptions of his illness by witnesses. 

See this BBC article: What was the truth about the madness of George III?