But an explanation I had never given a lot of thought to until now is the fact that I am an only child. And one of the most comforting books I have read is out this week - Only Child: How to Survive Being One, by Jill Pitkeathley and David Emerson. The authors have interviewed 60 only children and come up with enough common characteristics to make an only child feel part of one of a huge, unknown family. We are a growing band: today, five times as many people are choosing to have only one child compared to a decade ago.
Some shared characteristics are obvious. Most only children feel special, and many feel upset if they are not the centre of attention. They have been used to it, so anything less spells neglect. But at the same time we feel lonely. One 'only' who suffered from a permanent sense of isolation was quoted as saying: 'I shy from having friendships that are too close - I think I find it too claustrophobic.'
Another downside is that while you get all the attention and all the goodies, you get all the blame, too. Someone's broken mum's favourite piece of china? If it wasn't your parents, there's only one person it could have been - you. 'Sorry,' is a word 'onlies' learn to say a lot, even when they are not in the wrong.
We can be extremely sensitive to criticism. 'Because you have no experience of sharing criticism with other people, you feel it is cataclysmic,' said another 'only'.
Sharing is another problem area for 'onlies', 'ours' being a word virtually unknown in my vocabulary. While 'onlies' give without problems, lending can cause great stress. Like many 'onlies', never having had communal possessions, I am neurotically scrupulous about returning borrowed things.
Being surrounded by adult company so much of our time, we acquire a super-sophisticated gloss much sooner than others. At an early age we may be 'quite the little grown-up'. But because we don't have brothers and sisters to take off our rough edges, we're often emotional cripples who throw childish tantrums when we can't get our own way. 'I can be sensible in some ways but I'm very childish,'said one 'only'. 'I'm a sort of retard.'
Yeah, yeah, you with siblings may be saying. This is mere palmists' patter, stuff which applies to everyone. True, but I would argue that 'onlies' experience this feeling in spades. Apparently, most only children find it difficult to rebel as adolescents. Since they are often one against two, 'passive aggression' is the technique they may use, and carry on using into later life, to the fury of their partners.
One of the reasons so many of us are so immature is that we missed out on the 'rough and tumble' of family life - and frankly, just the phrase makes my toes curl with horror. As I child I was too shy to go to parties, and though I now rather enjoy the cool, social banter of big cocktail parties, offer me Sunday lunch in a big rumbustious home, with lots of plonk, kids, screaming, shouting and jollity and my knees turn to jelly. I am filled with a mixture of dread and envy. As for the teasing that goes on, for me it is a kind of torture. Like other 'onlies' I have no idea of telling whether it is affectionate or unkind, and tend to burst into tears or take umbrage.
I remember the stress of Christmas with my former in-laws, a huge and friendly family. I would cower in my room reading on my own to the point of rudeness. I used to think it was because I was cold and snobbish; now I think it was the sheer terror of being faced with the cut and thrust of family life, which I had never - and still have not - the social skills to deal with.
In later life, it may be much harder to make ourselves heard. As children, the tiniest change in voice or alteration of body language was instantly spotted as a change of mood, but in a big family you have to shout. And even now what I call being furious is met by friends who have siblings with cries of: 'You call that furious] But you hardly raised your voice]'
There is not a lot to be said for being an only child. Apart from rather burdensome assets such as being responsible, punctual, conscientious - and in some cases supremely self-possessed - most of our other characteristics are distinctly unlovable. We tend to be devious, and to eavesdrop - the authors' explanation for this being that we are so desperate to be in on things without being involved that we love feeding off other people's conversations. Certainly being an agony aunt is a form of eavesdropping. And to put the final nail in our coffin, it turns out Hitler and Stalin were both only children. Genghis Khan too, no doubt.
There is a sad quote at the end of the book: 'There hasn't been a day of my life when I haven't wished for a brother or a sister.' But it's good to know we're not the only lonely ones. True, it helps to make us feel a little less special; but a little less lonely, too.
'Only Child: How to Survive Being One' is published by Souvenir at pounds 10.99.