Many of the details regarding Hadrian's personal life are largely speculative, but one relationship in particular has long been the subject of extraordinary attention. Despite the emperor's close relations to his mother-in-law Matidia (Trajan's niece), his marital arrangement with Matidia's daughter Vibia Sabina is characterized as distant at best.
According to the Historia Augusta Hadrian was well known for various sexual escapades including with married women, which probably did little to endear him to his wife. More scandalous however is the insinuation that the emperor preferred adult men over women (which perhaps oddly enough according to modern sensibilities, was considered less appropriate than maintaining sexual relations with 'boys'). However, there was one young member of Hadrian's imperial court that received particular attention from the emperor and from historians.
Antinous, a handsome Bithynian born about AD 110 (about 34 years Hadrian's junior), had met the emperor at some point in his early to mid teens and certainly joined him on various expeditions throughout the imperial provinces. While traveling the Nile circa AD 130 Antinous died under what appears to have been mysterious circumstances. While Cassius Dio reported that accidental drowning was the cause of death, the Historia Augusta more dramatically suggested that the young man sacrificed himself willingly because various omens implied that his own shortened life would prolong that of Hadrian's.
Regardless of the circumstances, Hadrian's reaction was one of overwhelming grief. While there is the possibility that Antinous was simply a favored protege (this is admittedly unlikely) the extravagant honors heaped upon the dead young man were enormous. Cities in his name were founded, statues were erected all over the empire, and Antinous was worshipped in association with the gods Osiris, Bacchus and others, essentially making him a member of the Roman imperial cult.
Whatever the true circumstances regarding Antinous, when Hadrian eventually returned to Rome for the final time from his wide ranging tour of the Empire (including the debilitating Jewish wars) he was faced with the difficult task of finding an heir. Hadrian was about 60 years old by this time (AD 136) and was facing deteriorating health. Without legitimate children of his own (and by this time his wife had died) adoption was the only choice and Hadrian wanted the succession to be public knowledge rather than a repeat of the mysterious circumstances surrounding his own adoption by Trajan. His first choice, L. Ceionius Commodus, was a young man (though in poor health himself) of considerable political connection but one who lacked military experience or redeeming qualities of any great substance. Perhaps fortunately from a perspective of prosperity, Commodus died before Hadrian (AD 138) did and the search for an heir continued.
Hadrian's next choice, T. Aurelius Fulvius Boionius Arrius Antoninus, who would become known simply as Antoninus, was a man only 10 years Hadrian's junior. Antoninus was not only of a distinguished consular family but had achieved the consulship himself en route to obtaining a position among the aristocratic elite as the governor of Asia Minor. His adoption would prove to be another positive step in the course of the "adoptive emperors" era.
Hadrian took the adoption a step further, however, naming Lucius Aelius Aurelius Commodus (Lucius Verus) and Marcus Annius Verus (Marcus Aurelius) as joint heirs to Antoninus. Perhaps he feared the conspiracy that could grow in the absence of imperial stability or perhaps he was simply trying to ensure the quality administration of the empire, but whatever his reasoning these actions saw to that continued stability for the next 40 - 50 years (arguably ending with the death of Marcus Aurelius in AD 180 or even through the reign of Commodus in AD 193).
With the matter of succession settled in such a way as to continue the era of the "5 Good Emperors" Hadrian died at the age of 62, after a lengthy reign of 21 years, on July 10, 138 AD at his villa in Baiae. With his death, opponents within the Senate saw the opportunity to finally have revenge for various slights and transgressions against them over the course of Hadrian's reign. Attempts to condemn Hadrian's memory (as had been done to Domitian) were foiled by Antoninus as he played the part of honorable adopted son. His diligence in reversing the sentiment against Hadrian and having his adoptive father deified certainly played a role in earning him the name "Pius".