MARCUS AURELIUS: If only you had been born a man, what a Caesar you would have made.
The Aurelius family is a mess. Just about everybody is a better "son" than Commodus. These words are addressed to Lucilla, who proves herself a more adept politician than her evil brother.
MARCUS AURELIUS: You are the son that I should have had…now embrace me as my son.
Maximus and Lucilla are better children to Marcus Aurelius than Commodus, and he loves them more.
COMMODUS: Even then it was as if you didn't want me for your son.
We get the idea that part of the reason Commodus is so screwed up and so obsessed with being the center of attention is because he has felt unloved by his father. Marcus Aurelius hasn't given Commodus the necessary amount of paternal love.
MARCUS AURELIUS: Your faults as a son, is my failure as a father.
Marcus Aurelius takes responsibility for his failings as a father, and even asks for Commodus' forgiveness in this scene (his gestures suggest as much). It's not enough. His "failure as a father" ultimately leads to his death.
COMMODUS: I call it love. I'm their father. The people are my children. And I shall hold them to my bosom and embrace them tightly.
Again, lots of irony from Commodus. He unwittingly attempts to give the Romans the paternal love he was denied, but he really just wants to control them. Besides, the last guy he "embraced" ended up dead.
LUCILLA: My brother hates all the world, and you most of all…because my father loved you.
This is another clue to Commodus' anger and frustration. We've gotten hints throughout the movie that Marcus Aurelius loved Maximus more than Commodus. Lucilla knows that, and she knows this is why Commodus hates Maximus.
COMMODUS: You loved my father, I know, but so did I. That makes us brothers, doesn't it? Smile for me now, brother.
Commodus' idea of family is so warped. He thinks that just because he and Maximus both loved Marcus Aurelius, they're somehow family? Even if they are, what does it say about Commodus that he stabs his "brother" and then tries to kill him in the arena?