From what I can tell, it's their only way to keep up a monotonous relationship by perpetually proving to each other that they have a jokes worth of wit.... Hmmm....
Okay, theres something of Poeticromance's point that I feel like adding to. I think there is a difference between using your imagination and being witty. Wit seems to last as long as the subject in question has an irony to milk, while imagination is more constructive. So, I'm random as well, but not in the simple way that requires single jokes based off meager subjects. Unless, of course, those subjects are somewhat intellectual. Then it just gets really fun to make them ridiculously hilarious.