Hadrian had taken a liking to Marcus Aurelius early in life, enrolling him into the equestrian rank at the age of six, making him a priest of the Salian order at the age of eight and gave him the best possible education at that time. Hadrian’s health had begun to fade, and with Commodus, his main heir dead, he needed to find somebody else to cover his reign. Hadrian may have wanted Marcus Aurelius to succeed him initially, but he knew he was too young. And so Antonius Pius became the successor, but only by adopting Marcus Aurelius and Commodus’ orphaned son, Lucius Ceionius Commodus as his heirs.
In AD 139 Marcus Aurelius was made caesar, which was ‘junior emperor’ to Antonius Pius, and in AD 140, he gained the position of consul for the first time.
The senate preferred Marcus Aurelius, and so when Antonius Pius had died in AD 161, the senate actually sought to make Marcus sole emperor. Marcus Aurelius, however, insisted that the wills of both Hadrian and Antonius Pius were that he would joint-rule with Verus, his adopted brother. Because of this, there was a clean succession for Marcus Aurelius and Verus, as the senate and the people both favoured them.