If there's a serious side, it's implicit, and would be something like: the tragedy of our failures at relationships is that we can't rewind and escape the roles in which we've cast ourselves. Is amnesia a metaphor for the clean slate on which life might be rebuilt? O'Farrell's observations of our ridiculous personal quirks and foibles make for comedy on many levels.
Every marriage has its own San Andreas fault running right underneath, and even the slightest rumbling or tremor can be attributed to that basic fracture deep below the surface. The fault line might be 'You only married me because I was pregnant' or 'You're never there when I really need you', but most of the time these powerful forces remain suppressed. But then, from nowhere, the crockery starts to vibrate and a family picture will smash to the floor, and before you know it the subterranean tectonic plates have collided and the screaming measures 8.2 on the Richter Scale.