No great comedian is ever amused by himself. Billy Connelly could have been a great comedian had he not taken to collapsing hysterically during his own routines. The seal on David Brent's prattishness was his laughing at his own jokes. Then it turned out that Ricky Gervais, who created him, laughs at his own jokes too. Self-satisfaction is an unpardonable crime in a comedian because his role is to remind us that nothing is satisfactory. Hence the necessity of keeping a straight face. It affirms the seriousness of his calling. Which is to make people laugh, not because life is funny but because it isn't.
(Hat tip: The Awl)