In Batman's Dark Knight, almost all other characters are following certain rules, but the clown has been breaking other people's rules since he appeared, and even the clown has been trying to force Batman to break his last rule in the film, "Don't kill people".
Judging from the logic in the film, the clown's starting point is that rules are a kind of self-deception and self-restraint of human beings in civilized society, and these rules do not exist in the real world; The starting point of clown behavior is to force people to abandon the rules through various means, face the chaos and impermanence of the world, and face the "real" humanity at the same time.
So "why is it so serious" can be understood as a clown's ridicule of the rules, and the rules themselves are an illusion. Why should we take them seriously?
Extended data:
Nolan's version of the clown is a high IQ criminal who ignores everything. He regards life, matter and spirit cherished by universal values as dirt. Ignoring life, killing without blinking an eye. He burned the Qian Shan he owned regardless of material things. He ignored the spirit, firmly believed that people's hearts were fragile, dark and selfish, and played with people's hearts in applause.
The clown's mantra is to let the other person give up these cherished things, put down the importance and seriousness, and embrace madness and disorder. Another meaning of this mantra is that in Nolan's eyes, everything is a game.