Sometimes television critics, including me when I'm in a bad mood, complain about the loss of the great shared television experience, that feeling of sitting round the telly with friends and family and watching the same programme at the same time.

But television has never really been about sharing at all - it is about loneliness and always has been.

It's about one person pressing the on switch because that means there are voices and faces in the room with them. Television is a kind of someone else.

This might be why Alan Bennett - who was celebrated with a series of repeats last week on BBC Four - has always been so well suited to television because his greatest subject has always been loneliness.

It is what he writes about over and over again: The Lady In The Van was lonely, Hector in The History Boys was lonely, and almost every one of the characters in the Talking Heads series was lonely - sometimes so lonely it was hard to watch.

Last week, to mark Bennett's 80th birthday, a couple of the best of the Talking Heads were shown again and they included A Lady Of Letters (Tuesday, BBC Four, 9pm).

First shown in 1987, the monologue features Patricia Routledge as a woman called Irene who is obsessed with writing letters, usually to the authorities about what she considers to be breaches of etiquette.

One of her complaints is about staff smoking outside a crematorium. "Of course if I'd happened to be heartbroken, it would have been much worse," she says. "But I hardly knew the woman."

There were other delightful, funny, revealing little sentences like that one including a comment about letters from her MP ("He's Labour but it's always good notepaper and beautifully typed") but it was the sentences that said one thing and meant another that reminded me why Bennett is so good.

At one point, Irene looks out of her window at the family across the road.

"I see that we have a new couple moved in opposite," she says and there it was, exposed: all her longing for a domestic life, a husband and a child.

It was the loneliness again, revealed, exposed and made bittersweet and funny.

A couple of days before the showing of the Routledge play, the director Nicholas Hytner interviewed its creator for Alan Bennett At 80 (Saturday, BBC Four, 9pm) and suggested that as well as loneliness, the other great theme of his work is regret.

More often than not, said Hytner, Bennett's characters miss their opportunities rather than seize them, and the great man nodded at that one.

"The things you remember are the things you didn't do," he said.

The great skill of Bennett has been to tap into that sadness with jokes that hurt; he has made melancholia watchable by putting laughing and crying right up against each other, like they are in life.

But does this mean there are lessons for modern television?

Probably not. We now look in vain for real life in scripted and directed reality shows instead of looking at the fiction of Bennett, where real life always shows up, painfully, honestly and movingly.