First, a more general reason: Psychologically, it’s common for people to laugh in sad or uncomfortable situations. Clinical psychologist Dr. Abigael San explained in a 2021 Express article about why people laugh at funerals: “A funeral is a situation that many people find very difficult emotionally with everything that it represents. An expression of laughter can be what we call a manic defense, almost like a coping strategy but in a defensive way. Anything to divert the reality of what is happening and the reality of loss and human vulnerability and the end of life.”

However, when it actually comes to Daemon and his motives, it’s likely that something sinister is afoot. He laughs when the priest discusses the ties between families, but Daemon of course knows that there’s bad blood between the families, so he could be laughing at the lack of truth. Or, he might be realizing that he can now finally be with Rhaenyra.

Source: https://www.distractify.com/p/why-did-daemon-laugh-at-laenas-funeral