What caused John Stuart Mill’s mental crisis in 1826?

In 1826, John Stuart Mill experienced a mental crisis caused by a profound misgiving regarding the value of the goals he had set for his life.1

Raised under the rigorous and demanding tutelage of his father, James Mill, he had been trained as an intellectual machine designed to promote specific utilitarian doctrines. However, he began to feel that he was merely a "made man" set to "grind certain tunes," leading to a period of intense doubt and "gloomy solitude".12

This crisis forced him to confront the limitations of the narrow, strictly rationalistic upbringing he had received. As he worked through these doubts, he eventually emerged with a broader perspective on human happiness, developing a new appreciation for poetry, a more tolerant attitude toward controversy, and a rejection of the rigid sectarianism he had previously been taught.1