Because he often wore the shoe so called, to win the men's goodwill.
Ann Book I Chapter 41: Revolt in Germania. Germanicus' family

He was a man who masked a savage temper under an artful guise of self-restraint, and neither his mother's doom nor the banishment of his brothers extorted from him a single utterance.
Ann Book VI Chapter 20: Gaius Caesar

There never was a better slave or a worse master.
Ann Book VI Chapter 20: Gaius Caesar
By Passienus

Once too when Gaius Caesar in a casual conversation ridiculed Lucius Sulla, he predicted to him that he would have all Sulla's vices and none of his virtues
Ann Book VI Chapter 46: Succession of Tiberius
By Tiberius

At the same moment he embraced the younger of his two grandsons with a flood of tears, and, noting the savage face of the other, said, "You will slay this boy, and will be yourself slain by another
Ann Book VI Chapter 46: Succession of Tiberius
By Tiberius

Was it probable that, when Tiberius with his long experience of affairs was, under the influence of absolute power, wholly perverted and changed, Gaius Caesar, who had hardly completed his boyhood, was thoroughly ignorant and bred under the vilest training, would enter on a better course, with Macro for his guide, who having been selected for his superior wickedness, to crush Sejanus had by yet more numerous crimes been the scourge of the State?
Ann Book VI Chapter 48: Arruntius and Albucilla
By Lucius Arruntius

Even Gaius Caesar 's disordered intellect did not wholly mar his faculty of speech.
Ann Book XIII Chapter 3: The funeral of Claudius

It was well said of him that no one had ever been a better slave or a worse master
Stn Caligula, Chapter 10: Caligula on Capri.

He could not control his natural cruelty and viciousness, but he was a most eager witness of the tortures and executions of those who suffered punishment revelling at night in gluttony and adultery.
Stn Caligula, Chapter 11: Caligula on Capri (cont.)

He used to say now and then that to allow Gaius to live would prove the ruin of himself and of all men, and that he was rearing a viper for the Roman people and a Phaethon for the world.
Stn Caligula, Chapter 11: Caligula on Capri (cont.)
By Tiberius

He began from that time on to lay claim to divine majesty; for after giving orders that such statues of the gods as were especially famous for their sanctity or their artistic merit, including that of Jupiter of Olympia, should be brought from Greece, in order to remove their heads and put his own in their place.
Stn Caligula, Chapter 22: The Divine Caligula.

He lived in habitual incest with all his sisters.
Stn Caligula, Chapter 24: Caligula and Drusilla.

no evidence convinced him so positively that she was sprung from his own loins as her savage temper, which was even then so violent that she would try to scratch the faces and eyes of the little children who played with her.
Stn Caligula, Chapter 25: Caligula as a monster (Cont.)

He had the manager of his gladiatorial shows and beast-baitings beaten with chains in his presence for several successive days, and would not kill him until he was disgusted at the stench of his putrefied brain.
Stn Caligula, Chapter 27: Caligula as a monster (Cont.)

Saying that the rule of Augustus had been made famous by the Varus massacre and that of Tiberius by the collapse of the amphitheatre at Fidenae, while his own was threatened with oblivion because of its prosperity
Stn Caligula, Chapter 31: Caligula as a monster (Cont.)

He would bathe in hot or cold perfumed oils, drink pearls of great price dissolved in vinegarů
Stn Caligula, Chapter 37: Caligula as a monster (Cont.)

He wrote besides to his financial agents to prepare for a triumph at the smallest possible cost, but on a grander scale than had ever before been known, since the goods of all were at their disposal.
Stn Caligula, Chapter 47: Military affairs of Caligula (Cont.)

There was found besides a great chest full of divers kinds of poisons, which they say were later thrown into the sea by Claudius and so infected was it as to kill the fish, which were thrown up by the tide upon the neighboring shores.
Stn Caligula, Chapter 49: Military affairs of Caligula (Cont.)

His hair [was] thin and entirely gone on the top of his head, though his body was hairy.
Stn Caligula, Chapter 50: His countenance.

he never rested more than three hours at night, and even for that length of time he did not sleep quietly, but was terrified by strange apparitions
Stn Caligula, Chapter 50: His countenance.

For this man, who so utterly despised the gods, was wont at the slightest thunder and lightning to shut his eyes, to muffle up his head, and if they increased, to leap from his bed and hide under it.
Stn Caligula, Chapter 51: His fears.

Besides a stall of marble, a manger of ivory, purple blankets and a collar of precious stones he even gave this horse [Incitatus] a house, a troop of slaves and furniture, for the more elegant entertainment of the guests invited in his name; and it is also said that he planned to make him consul.
Stn Caligula, Chapter 55: Caligula and the circus.