Why did Edward Gibbon consider the age of the Five Good Emperors a happy period in history?

In his monumental work, The History of the Decline and Fall of the Roman Empire, the British historian Edward Gibbon identified the era of the Five Good Emperors—spanning from the death of Domitian in 96 CE to the accession of Commodus in 180 CE—as the period in which the condition of the human race was most "happy and prosperous."1

Gibbon viewed this era as a high point of human history characterized by internal peace, security, and the effective reconciliation of peoples throughout the Roman Empire. During this time, the empire experienced significant consolidation and expansion, supported by a uniform provincial system and a sense of stability under the rule of Nerva, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius. For Gibbon, this period represented a pinnacle of civilization before the onset of the moral and political decay he traced throughout the remainder of his narrative.1234