The accession of the 19 year old Commodus upon the death of his father Marcus Aurelius on March 17, AD 180 is often considered a catalyst that sparked the initial decline of the Roman Empire. Whether or not the assertion is true and Commodus acted as a catalyst, or if he was simply a piece of an evolving and dynamic Roman puzzle is a matter of perspective and interpretation. It's interesting that the death of Marcus Aurelius is considered the end of the "Pax Romana" (The Roman Peace) but the reign of Commodus was largely as peaceful as that of Antoninus Pius a half century earlier. While he was immensely popular with the common people and legions, his reputation as a contributor to the decline of the empire was largely dependent upon a poor relationship with the Senate. Megalomaniacal behavior, irreverence for the institution of the Senate and a Sulla like affinity for proscription earned the lasting enmity of that social order that produced the historians.
Lucius Aelius Aurelius Commodus (later Marcus Aurelius Commodus Antoninus upon accession) was born August 31 AD 161 at Lanuvium, a small town in the outskirts of "metropolitan" Rome. He was the second to last of 13 children born to the Emperor Marcus Aurelius and Faustina the Younger. Of these children, 7 were boys but only Commodus seems to have survived beyond young adulthood. His father had been emperor for only a few months (March, AD 161) prior to his birth, but Commodus had been marked for succession very early. He was officially named Caesar (or heir) at the age of five and had begun a typical aristocratic education. Such an example of direct hereditary accession had not occurred since the Flavian dynasty (Vespasian, Titus and Domitian) nearly a century prior. While the previous emperors (Trajan, Hadrian, Antoninus, Marcus Aurelius and Lucius Verus) had been cultivated through adoptive succession, this can at least be partly attributed to the fact that these men did not have sons of their own. While Marcus Aurelius is criticized for appointing his son as heir in contrast to his predecessors rather than adopting a qualified candidate, it's impossible to determine the course of events had circumstances been different.
The ancients (i.e. Herodian, Cassius Dio and the Historia Augustus) speculate on the manner of Commodus' personality and set him up almost as a pre-destined evil entity. Whether the "evils" of Commodus as reported were due to later events in which he alienated himself from the aristocracy, or were in fact representative of Commodus' nature and upbringing in a condition of supreme authority is clearly open to perspective and interpretation. Whatever the foundations of enmity between Emperor and nobility, be it a clear behavior and personality issues beginning in his youth or rather the nature of his later administration, there remains a consensus of distaste from the ancients. Dio opens his book 73 on Commodus thusly: "This man was not naturally wicked, but, on the contrary, as guileless as any man that ever lived. His great simplicity, however, together with his cowardice, made him the slave of his companions, and it was through them that he at first, out of ignorance, missed the better life and then was led on into lustful and cruel habits, which soon became second nature. And this, I think, Marcus clearly perceived beforehand. Commodus was nineteen years old when his father died, leaving him many guardians, among whom were numbered the best men of the senate. But their suggestions and counsels Commodus rejected, and after making a truce with the barbarians he rushed to Rome; for he hated all exertion and craved the comfortable life of the city."
While there is speculation that Marcus Aurelius feared for the future of the empire because of the mannerisms of his son, his actions do not necessarily correspond to this notion. At the age of 16 years Commodus was appointed to the high station of co-Augustus, though his father did maintain clear ultimate authority. While this was done in part to illustrate the well defined nature of succession after the ill-fated revolt of Avidius Cassius in Syria, for much of his childhood and early adulthood, Commodus had already accompanied his father while on campaign along the Danube. Without question he was groomed as successor and was even given the military training his father never had and by AD 178 was leading legions in battle against the Quadi and Marcomanni.
When Commodus was only 19 years of age, Marcus Aurelius (just shy of his 59th birthday) died in AD 180 and the control of the empire was left to the a young man with a great deal to prove. Commodus immediately began to treat with the Marcomanni in order to bring the decade's long wars of his father to a close. While this was unpopular in many circles it may not have been as unpopular with the legions and common citizen classes as generally reported. While there may have been a sense of a job left unfinished and his father's wishes to see the border lands transformed into Romanized buffer provinces were left unfulfilled, the war coupled with the Antonine plague had drained the treasury and the manpower pools. Commodus understood that the war had been devastating despite its accomplishments in ensuring peace along the Danube for the better part of the next generation. In addition and perhaps more importantly, as a young man, despite his grooming under his popular father, he also surely understood the need to return to Rome in triumph to secure his possibly tenuous position as successor. Only a few years removed from the revolt of Cassius, for Commodus to have risked alienating his army by ending the war early, the one force capable of securing any bid for succession, seems unfathomable. Rather it's quite likely judging by the initial popularity of the youthful princeps, that the legions were all too happy to have peace with their persistently dangerous Germanic neighbors.
The bitterness expressed by the aristocracy over Commodus' handling of securing that peace seems to be the catalyst that sparked initial dislike. In his haste to return to Rome, Commodus seemingly made several unpopular concessions (release of prisoners, annual levies and grain procurements, etc.) which possibly could've been avoided if there was no need for haste. In order to secure his position, he sacrificed some of the hard fought victories of the past 20 years, and it's difficult to determine this as brash, dismissive exuberance or wise policy. It had been quite established that the ability of the empire to maintain borders outside natural defensible positions such as the Danube was difficult at best (Augustus/Tiberius in Germania and Trajan/Hadrian in the east and Dacia). Commodus may have understood that continuing the war would be costly and eventually futile or he may have truly just wished to return to Rome in glory. Regardless of the truth, he seems to have never been forgiven by the writers of history. In fact, Cassius Dio blames the emperor's haste simply on the notion that "he hated all exertion and craved the comfortable life of the city."