Howard Jacobson: There's only one thing a sense of humour will get you – and that's into trouble

I'm under orders to stop being entertaining, or if that sounds vain, to stop trying to be entertaining. Doctor's orders. Wife's orders. Friends' orders. Enough with the jokes. And don't say, "Enough with the jokes, already", because that sounds as though you're still trying to be entertaining.

At one level this isn't difficult to obey. Nothing is that funny any more. The how many houses has Jacqui Smith fiasco – husband watching blue movies in the one in Redditch, policemen counting the number of nights she is at home in the one in south London, something about rooming with her sister, the feeling you get that at any moment she will claim she has been rooming with you – reader, it's no longer amusing. Ditto Gordon Brown's daily discomfiture. Ditto the idea of Cameron doing any better. Ditto a new novel by Dan Brown. Ditto – while we're on books, and what could be less amusing than books – the ex-X Factor winner Leona Lewis signing copies of her autobiography at Waterstones.

Her what? Her autobiography. Leona Lewis is 24 and she, or someone, has written her autobiography. Should be funny but isn't. Should be funny that countless thousands of young girls will be buying it to learn how they too can become an ex-X Factor winner and write, or have someone else write, their autobiographies, but it isn't.

You'll have examples of your own. Of what should be funny but isn't. Maybe you know a banker on a bonus. Maybe you live next door to Nick Griffin. Maybe you're waiting for a letter. Should be, but isn't.

But my being told to stop telling jokes doesn't really have anything to do with whether anything is funny or not. The best jokes, anyway, illuminate what isn't funny in the slightest. Think Hamlet in the graveyard, discussing stand-up with the skull of Yorick. Get you to my lady's chamber, tell her, let her paint an inch thick, to this favour she must come – make her laugh at that. Death and dissolution: those are the true province of comedy.

Hence my trying to be entertaining every time I visit the doctor. "So what's wrong with you?" my wife asks when I get back. "Oh, nothing," I reply. "What do you mean nothing? What did he say?" I hang my head. The truth is I can't remember what he said. And why is that? Because I wasn't listening. And why was that? Because I was entertaining him.

It seems a reasonable thing to do while I'm doing it. Into his surgery of no hope his patients troop, one morbid case after another, diseased, distraught, self-obsessed. He needs someone to lighten his morning. And given where his hand is, I need someone to lighten mine. So I do wit. When his hand starts shaking I wish I hadn't. But I have this idea that if I can make him feel more cheerful while he's exploring me he won't find anything. "Ha, ha, ha, guess what you've got" seems unlikely, don't you think? So without exactly having formulated a theory of laughter and good health, I must believe that as long as we're both having fun there won't be any malignancies in my body. But that means I don't concentrate on what he says and can't remember what he reckons is or isn't wrong with me when I get home.

My wife now forbids me entertaining anybody who comes to the house on business. Particularly those who meticulously charge us by the minute. "I have noticed," I told the man who came to fix the microwave last week, "that you haven't deducted any time for laughing at my jokes." Whereupon he fell into more paroxysms of feigned laughter but still didn't stop the clock. I wondered why my wife was glaring at me. "You can pay for that one," she said afterwards.

What the hell! Four and a half hours to fix a switch at 75 smackers an hour starting from when he rang the doorbell. Worth it for the comedy alone.

Recently my wife and I had a blood test together. Not a fetish, just something we needed to do to save time. The phlebotomist wore a trainee badge. "So how many have you done?" I asked. My wife glared. "Forty-eight," he said. "And how many of those have lived?" My wife glared again. He smiled. "All, I hope," he said. "Until now, maybe." He was doing my wife at the time and in the exchange of banter failed to press hard enough on the square of lint that goes over the puncture. As a consequence a purple bruise the size of a tennis balls spreads over her arm. My fault. I distract a trainee phlebotomist with a joke and my wife is disfigured.

"But at least he's had a more than usually entertaining morning," I tell her.

In fact, those you entertain don't always thank you. A number of years ago I stumbled into the Brooks Brothers store in Regent Street, fancying a pair of canary yellow Ivy League cord trousers I'd seen in the window and was surprised to find the entire sales staff lined up, as though to greet the Queen, but evidently, on this occasion, lined up to greet me. Were they familiar with this column? My novels? Had I found readers on a scale only Leona Lewis dare dream of?

"Highly appreciated but not necessary, guys," I said, expecting applause. Not a one of them moved a muscle. "Stand easy," I told them. Not a flicker. "Do I just measure my own inside leg, then, or what?" I asked. Still no acknowledgment, unless you call the icy contempt of 15 stuffy Brooks Brothers salespersons an acknowledgement.

It was only as I was leaving the shop without canary cords that the chimes of bells sounded and I learnt that the staff had been marking the anniversary of 9/11 with two minutes' silence. The rest of Regent Street had been doing the same. The only unquiet thing anywhere was me. The entertainer.

I suppose it's terror on my part that does it. As long as you keep making jokes you don't have to listen to anybody else. What was it George Eliot wrote in Middlemarch about the necessity of a certain kind of moral deafness, without which we would die of the roar which lies on the other side of silence? Which of us really wants to hear what the doctor has to tell us? Or what celebrity has to advertise, or what the ideologue demands, or what the greedy have to say in their own defence? Better to drown it all out with something funny. If you can think of something funny.