Liber Pater

Having studied classical history for a few years now, the most satisfying answer to this question to me is that the Julio-Claudian dynasty all shared a hereditary disease. Some lists of proposed illnesses exist on independent sites, I feel that if this were true it could answer many questions we have about the actions of these emperors. This theory has reasonably good footing as well as Suetonius details that after his first 'good' year, he should be called Gaius the monster and not Gaius the Emperor, this is shortly after he falls into a Coma and so perhaps there is room for this idea of a mystery illness ? Regardless, it could also be nurture over nature as he did live a traumatic life, even by Roman standards. He had likely been groomed by Tiberius on Capri, lived on a military camp (Hence his nickname Calligula or 'Little boots') and so was surrounded by violence as he grew up and was subjected to the many cruelties that took place in the treason trials. To answer your question, no one truly knows however I feel the most applicable reasoning comes from the idea of a traumatic life or simply an illness that had been present in all of his dynasty.