You may not know it, but today is Eeyore's birthday. And if there's one thing that upsets Eeyore more than anything, it's people forgetting his birthday. Traditionally Eeyore might receive a popped balloon from Piglet, or an empty honey jar from Pooh. But since AA Milne's gloomy donkey turns 140 today – which is Very Old Indeed – I think we should surprise him by celebrating his status as a truly great literary character.

The Winnie-the-Pooh stories are part of the fabric of our lives. We grow up reading them, then we read them to our children. But while each character is loveable, Eeyore seems to have a special place in our hearts. We are drawn helplessly towards him; we recognise something deeply human in his gloomy outlook. His sadness is our sadness. He's an Everyman; an Every-donkey.

In literary terms, Eeyore is the archetypal outsider. The other animals – Pooh, Piglet, Owl and the rest – dwell happily within Hundred Acre Wood, knocking on each others' doors, having tea and embarking on adventures. But not Eeyore. He lives on the other side of the stream in his Gloomy Place – marked on the map as "Rather Boggy and Sad". Rather than venture out to see others, he waits for them to pass through his field, which doesn't happen often. "I have my friends," he notes ruefully. "Somebody spoke to me only yesterday. And was it last week or the week before that Rabbit bumped into me and said 'Bother!' The Social Round. Always something going on."

So what does Eeyore spend most of his time doing? Like all great outsiders, he Thinks – and he takes great pains to distinguish himself from the other animals for this. ("They haven't got Brains, any of them, only grey fluff that's blown into their heads by mistake..."). There he is in his lonely corner of the forest, sometimes thinking sadly to himself, "Why?", and sometimes "Wherefore?" – and sometimes not quite knowing what he's thinking at all. While the other animals amble contentedly through their daily lives, Eeyore wrestles with these questions alone.

The tragedy is that all this thinking doesn't make Eeyore happy. As Benjamin Hoff has argued in his marvellous book The Tao of Pooh, Eeyore can't enjoy life because his mind is clouded with thoughts that cut him off from the world around him – which, in the case of Hundred Acre Wood, is a beautiful one. He can't live the simple, spontaneous and joyful life of Pooh – a Bear of Very Little Brain, maybe, but the happiest character in the wood because of it.

So the riddle of Eeyore is this: why, despite his many failings, do we love him so much?

I think it's partly because, as well as being the most depressing individual in the Pooh books, he is also the funniest. Melancholy often teeters on the brink of absurdity, and Eeyore regularly falls over the edge. Take the classic scene in The House at Pooh Corner when Eeyore tumbles into the stream after the irrepressible Tigger bounces up behind him and takes him by surprise. The image of Eeyore, floating around in circles with his feet in the air, trying to maintain his sombre demeanour, is desperately funny and sad.

And then there's his truly glorious sarcasm (of which there are too many instances to catalogue here), whose hilarity is heightened further by the way that it sails straight over the other characters' heads.

But the key thing that makes Eeyore a great character is that essential literary ingredient: conflict. Eeyore is profoundly conflicted. He craves love – indeed, he's always lamenting his outsider status – but he struggles to give and receive it. When it's offered to him, he puts out his hoof and waves it away. There are many occasions when Pooh and Piglet, who love Eeyore unconditionally, pay him a visit only to be greeted with a barrage of sarcasm. Nowhere is this more poignantly displayed than the scene in The House at Pooh Corner where Piglet realises that Eeyore has never had a bunch of violets picked for him. When he finds Eeyore to deliver the bunch, however, he gets shooed away. "Tomorrow," says Eeyore. "Or the next day."

It's this conflict that humanises Eeyore, and makes his plight a sympathetic one. Despite his unyielding misery – or perhaps because of it – we all love Eeyore, so let's celebrate his birthday. I'm sure it would make him happy: eyes down, tail swishing, mumbling through a mouthful of thistles... "Thanks for noticing me."