Simply put, the previous emperors had no heirs so they adopted popular and effective leaders or groomed them from a young age to take over the role of emperor. Nerva was selected late in his life by the senate to fill the void after the assassination of Domition and the end of the Flavian dynasty. Nerva himself was 65 when he was elected with no children to follow him. As well he was embroiled in civil unrest due to economic turmoil and plague. He adopted Trajan as he was a famous and respected general who could command the unruly armies. Effectively Nerva was emperor to pick a successor and died shortly after.

Trajan as well did not have any children and named Hadrian his successor through adoption. Hadrian did not have any children and was renown for his public tryst with his male lover Antinous. Hadrian adopted Antoninus Pius shortly before his death under the condition that Lucious Verus and Marcus Aurelius be adopted by Antoninus to be groomed as successors.

Antoninus Pius rose to the throne at the age of 61 with no surviving male children and had agreed to groom two young men as heirs to the throne. Both Lucius Verus and Marcus Aurelius served as co emperors until the death of Verus in AD 169 after which Marcus Aurelius ruled alone.

Marcus Aurelius was the first emperor since the beginning og the Flavian dynasty to have a legitimate heir to the throne. Commodes was groomed by a young age to succeed his father however his personality and possible mental health concerns prevented him from becoming a decent emperor. The adoptive emperors were an interlude rather than the norm of imperial dynastic politics with the adoptions being of necessity due to a lack of an heir.

I will not presume to tell you what Marcus Aurelius thought however it is plain he educated his son to succeed him. This would have been common practice in the preceding dynasties however he broke the practice of adoptive emperors with a substandard emperor leaving that blemish and historians to question his motives.

Cassius Dio: Historia Romana