Reddit user Rain 12913 claims to be a psychologist and offered analysis and diagnosis for the character Cersei Lannister on Game of Thrones. She's a fictional character, which the psychologist was quick to point out. But it's fun to use a real life diagnosis to explain the driving motivations of Cersei. It turns out she has a personality disorder. The fact that she cares mostly about herself—or people with personalities that are nearly identical to hers—makes her a classic narcissist. As always, there are a whole bunch of spoilers for the series ahead.
To be certain, Cersei would have plenty of certifiable mental health issues. Sleeping with your brother would be a paraphilic disorder, and a penchant for wild fire and pyromania would be classified under conduct disorder. But here's an analysis of her emotional responses and actions that make her a narcissist:
Cersei is a classic narcissist. As such, she lacks the ability to truly empathize with others. Despite this obvious reality, people seem to be falling into the trap of thinking that Cersei really does genuinely love her brother and her (late) children. While she certainly says that she does quite a bit, and while her behavior may seem to suggest that she does, it is highly unlikely that such a narcissistic character is capable of true love.