The rhythm of life is a powerful beat

Why can't I stop making childish jokes about Theo's mum? Especially when she's been dead for 20 years?
The sad truth is, that were I to get to the bottom of this question - like I like to get to the bottom of Theo's mum - I fear something unpleasant about me would be revealed.
The simple explanation for the jokes has two central facets: i) our reliance, Theo and I, on that North American argot which requires you to insult one another's mother with a degree of frequency (actually, this only applies to me); ii) my miserable bastard of a memory, which keeps forgetting that Theo's mum is dead, even though she's been dead for twenty years. I think the second facet is the more important one to explore.
The thing is, while I can blame the ravages of age and intoxication for laying waste to my memory, its cells and receptors, the truth is I can still recall the name of the Bolton midfielder currently on-loan from Liverpool (Danny Guthrie) and the name of the Peter Serafinowicz character who's a pastiche of those injury claim lawyers who make their own tv ads (Bryan Butterfield). I can even remember how to spell Peter Serafinowicz.
So what is it that means I remember those inconsequential facts but forget the tragic passing of Theo's much-loved mother? Has he just not articulated the fact clearly enough to me? Deep down, do I suspect he's lying about it? Do I harbour the suspicion that he never had a mum and was, in actual fact, raised from seed? Or am I simply callous as to the important details of my friend's life?
On reflection, if I had to pick an explanation, I'd probably go for the one about the seed.