I hated it as a child - the only child of older parents - and I hate it now. I had lots of friends but the sheer, overwhelming loneliness was horrible. It was particularly bad when my dad died 20 years ago and my mum tried to get me to move back to her because she had nobody else, and even worse two years ago when my mum was critically ill (and then died) and there was nobody to split the worry with. Now I hate that I have no parents and no siblings, that my children have no aunts or uncles on my side, that there is nobody to share childhood memories with.
When DH's parents died he was able to split both the worry and the practical work with his siblings. Not all sibs can do that, but many manage it to some extent. It's not about being your 'best friend' with a sib, that would be a bit weird, but simply having someone in the same position as you, someone who deflected from your parents' attention when you were growing up, someone who changed the family dynamic from being two adults and one child, someone who understood when you wanted to whinge about something at home.
"Surely you can't miss what you have never had?"
Isn't that a bit of a silly thing to say? Of course you can feel a sense of loss for what you've never had, if you can see what others have. What do you think women who can't have children at all are feeling if not that same sense of something being absent?
When you have an only child you take a risk - some kids seem to love it, others hate it (you only need to see the split opinions here). You need to accept that risk, just as you accept a risk simply by having any child, or the risk if you have two that they will utterly hate each other one day. There are no guarantees with kids.