With any ancient historical figure as sensationalized as Emperor Caligula, it won't do to consult only one source. In addition to the 2 primary ancient sources (Suetonius & Cassius Dio), I'm also reading all three of these modern accounts by historians Arther Ferrill, Aloys Winterling, and Anthony A Barrett.

Since most modern people's ideas of Caligula are based on what's essentially a porno film (which itself was based on Suetonius, who was basically as reliable as a tabloid in ancient Rome), and since every monster is still human, I intend to give you a much more balanced look into what made Caligula what he became.

The combination of being the adored as the darling of Rome among his father's troops in Germania wearing his little soldier's boots ("Caligula" means "little boots") from infancy, and the trauma of losing his father at age 7, then his mother and 2 older brothers, and having to walk on eggshells for years in close proximity to Tiberius who had likely had his family killed, seem to have created all the conditions that often lead to borderline personality disorder. 

Considering his immense popularity initially, and the fact that upon losing his favorite sister Drusilla within the first year of his reign, his behavior took a drastic turn for the worse, it seems likely to be a poisonous combination of borderline personality disorder, very likely PTSD, and accelerating megalomania from his position of essentially boundless power - and having lost the last person in his life that he had held dear to him, a willingness to take his personal pain out on the whole world.