My two-year-old daughter made me cry the other night.

It came as a bit of a surprise, because I’m not really that emotional a person. Over the years I’ve built up a thick skin – it’s the only way to survive being a square peg in a world of round holes. My moods tend to vary between melancholy, discontentment and ennui, so I rarely reach the extremes of feeling that lead to tears, good or bad. Funerals? Nothing. Weddings? Nothing. The birth of my kids? Meh.

But then, there is a chink in my armour. Toy Story 3 made me weep in the cinema, My Girl just kills me, and who doesn’t cry at Marley & Me (besides cat lovers)? I can’t walk past a child’s gravestone without welling up, and last year I even cried at a book, Karen Joy Fowler’s We Are All Completely Beside Ourselves, in a scene where a chimpanzee begs to be allowed to go home. All of which goes to show, give me the bittersweet juxtaposition of innocence and sorrow, and you can pierce right to the heart of me.

I first noticed this weakness about twenty years ago, at Land’s End in Cornwall. A little boy was running with his brand new toy sword from the gift shop when he tripped and fell and – SNAP! – the blade broke at the hilt. The look on that boy’s face – the dawning realisation of what had happened, the switch from innocent joy to infinite sorrow as life’s hard truths hit home, and then the tears of impotent despair at the discovery that some things once broken cannot be fixed – it broke my heart.

I mean, sure, it was just a plastic sword costing a couple of quid and his parents could have bought him another one in a heartbeat – hardly a life-or-death experience. But that boy’s face haunted me for weeks after, because in the innocent, uncontrolled emotional state of a child, unable to weigh up comparative value or process cause, effect and consequence, and living solely in the moment, it is life-or-death. Children and animals, their simplicity of thought and emotion, their purity – when they suffer, when they’re sad, when they’re in pain and when they die, it cuts through every barrier I put up to protect myself.

Unfortunately, my toddler is right at that point in her social and emotional development where innocence and sorrow come into contact several times a day.

The evening she made me cry, I picked her up from nursery as usual. It’s always nigh identical to that scene in The Railway Children – she sees me, stops stock still in awe, and then she shouts, ‘Daddy, it’s my daddy!’ and runs towards me, her face filled with elation, leaving me just enough time to drop to one knee before she slams into me and throws her arms around me. So excited to show me what she’s been doing, so proud to show me off to the ladies at nursery – ‘My daddy,’ she says, ‘This my daddy.’

She’s the last to be picked up, after dark, so for half an hour she gets to hang with the grown-ups. I know it makes her feel special. She gets such a look of well-being on her little face as she puts on her school bag like a big girl, waves to the ladies all cocky because she’s heading home to mummy and her little sister. This night was no different but for one thing.

She tripped as she stepped over the threshold, stumbled down the wheelchair ramp and face-planted into the mud.

I stood her up, her hands, coat and face black with dirt. The women from the nursery appeared in the doorway and in the light spilling out past them I saw my little girl’s face – the shock giving way to embarrassment and humiliation as she fought back the tears, struggling to keep control. I told her it’s okay and she’s very brave, but it was all too much and suddenly she was wailing and burying her face in my side so nobody could see her. Ultimate joy to ultimate misery in under ten seconds, her special, sacred moment destroyed. Broke my heart.

But that wasn’t what made me cry.

On the way home, to distract her from her misery, I asked her who had been there today. Turns out it was Tilly, Hugo, Sebastian, Rufus (yes, I know – we’re only a Tarquin away from winning Pretentious-name Bingo), and a new one for me – Jasper.

‘Who’s Jasper?’ I asked.

‘My best friend,’ she replied. Too cute!

Then I asked her what she’d been up to. ‘Me sing Twinkle, Twinkle with my friends.’ Oh my gosh, the sweetest thing ever. But it still didn’t make me cry. No, that came after dinner when I was bathing her.

She was sticking the foam letters to the side of the tub – ‘This mummy,’ she’d say, and ‘This Granny,’ and ‘This Poppa.’ Then she put three together, pointed at the middle one, and said, ‘This daddy.’

‘Who’s this, then?’ I asked, pointing at the figure beside me.

‘This daddy’s friend,’ she replied, and pointed to the other; ‘and this daddy’s friend.’ And then she put another one beside them and said, ‘And this daddy’s best friend.’

And that’s when I cried.

As a master at acting ‘normal’, I hid it well. This is particularly important because my toddler has become very sensitive to other people’s feelings. She’s always asking if mummy’s sad, or if daddy’s sad, and the other night she woke up sobbing because she’d had a dream in which mummy was very sad. So I wiped my eyes, endured that prickly feeling at the top of my nose, and got on with it.

But why did I cry? The juxtaposition of innocence and sorrow.

As somebody with autism, friendship is something I always desperately wanted but was never able to have. I struggle to understand or connect with other people. When someone wants to be my friend, I become paranoid and push them away. When I want someone to be my friend, I approach it so cautiously I miss the opportunity. I don’t know how to make, keep and manage friendships, and I only have the mental energy or focus to sustain one friend or partner at a time. As I’m married, this means I don’t have the social resources to have any friends – no close ones anyway. It’s the way I’m built. It’s one of those things.

But that doesn’t make it any less painful, and it doesn’t mean I’m not desperately lonely.

My daughter has already realised the importance of friendship. Watching her making friends is a wonderful relief, because she is not like me. A bittersweet relief, as one day she’ll learn that daddy doesn’t have any friends, and she won’t understand why, and she’ll be sad, because even though I won’t show it, she’ll know that I’m sad too. Because friendship is important regardless of who you are.

Where do innocence and sorrow factor into this? Her innocence; my sorrow.

That’s how my toddler made me cry.