In order for people with siblings to imagine being an only child, they effectively have to imagine their siblings out of existence. Very few people are willing or able to do that consistently. Sure, you have days when you'd gladly sell your sister to the Mongols, but most adults have fewer and fewer of those days as time goes on. And even if you imagine your sibs out of existence, you can't really see how it would be different, how your relationship with parents and friends and relatives would change.
It's just as impossible for us to really imagine what it would be like to have had siblings. We can try to make up people whole cloth, but even with the people we already know, it's hard to say what--for example--my relationship with my mother would have been if I'd had an older sister or a younger brother, to say nothing of either of our relationship with that hypothetical sibling. It's a multi-body problem. It's too complex. We just can't tell how it'd come out. You can't say, "If you had a sibling, someone else would share your love of _______," because odds are at least as good that the sib in question would hate _______.
I'm not just an only child but also an only grandchild on one side. We're estranged with my surviving grandparent on the other side (due to his being a lying ass and a coward), but even if we hadn't been, my aunt didn't adopt my one cousin and bear my other until I was 14, so I was very close to an only grandchild there, too. And I'm close, as I've said in other contexts, with great-aunts and -uncles who don't have grandchildren of their own, and with aunts, uncles, generations-older cousins with no children. So it feels to me like I'm the point of an inverted pyramid. I've always known that there will be no one else to look after my folks and my godfathers and so on when they are old, to say nothing of my grands and my great-aunts and -uncles. I was immensely relieved to have a second cousin in my generation on my dad's side of the family, because that accounted for five fewer old people in my care.
I don't know how to make it clear that I don't resent that, but I really don't. It's how things are. You get issued some future old people at birth and others you pick up through the bonds of marriage or affection, and you take care of each other. That's what people do. You pick up the small ones when they fall down and feed them berries and sing to them, and you make sure the old ones are getting their walks shoveled and occasionally chide them about vitamins and make sure they're comfortable with their living situation. (You can sing to them and give them berries, too, if you want.) This is how the world works. Some of us have more children than others, and some have more old people than others, and I've always known I would be in the latter and not the former group. Some of us take care of more people our own age, too, or take care of them more intensely. It's how things are.
That part may just be me. It's hard to spot chicken and egg here. Did I grow up with a "caretaker personality" because I was always aware that I would have more people to take care of, or did the awareness come from that personality trait? I don't know.
Being an only means that I can't take people's presence in my life for granted. When you're an only, no one has to put up with you on a roughly equal basis. No one is stuck with you but your parents, and everyone knows that's not the same. One of the books I read about onlies suggested that many of us make more effort to stay in touch with people because we're well aware that if we don't make an effort, other people may just wander off. (Others onlies are perfectly fine with that and are hollering, "Don't let the door hit you on the way out!" after them as they wander.) People talk about onlies being selfish, but if you don't share with your brother, you get sent to your room, and your brother is still around the next time. If you don't share with your friends, they go home, and they don't want to come back next time.
Being an only is like being one of thirteen kids in that perfect strangers will feel justified in criticizing your parents' family planning choices to you from the time you're about five years old. They will inquire after your parents' fertility, your parents' sanity, your parents' moral fiber; they will assume -- and tell you they are assuming -- that you must be really spoiled. Under what other circumstances would you inform someone who was behaving reasonably that she must be really spoiled? This is just rude. But should a kid learn that adults are sometimes rude and unpleasant, and how to deal with it without being rude and unpleasant? Yep. And is a kid going to have to learn to deal with it on one topic or another? Again: yep.
I have never once wanted siblings. Not for a nanosecond. Not for a femtosecond. Never.
A lot of the positive things I can say about being an only are things that aren't exclusive to being an only. I am, for example, happy with only myself for company. I learned to entertain myself at an extremely young age. Do some people with siblings have that skill? Sure. Do some other onlies lack it? Definitely. But there's something of a correlation. I felt more comfortable with adults earlier than most kids. I've been very close with older relatives. I don't feel like my goals are in comparison with other people but rather are in comparison with what I want to achieve and am capable of achieving. All these things can happen in multi-kid families, but they happen differently.
I feel like if I say, "I'm glad I'm an only because of X," I will immediately have people with siblings popping up to say that they had X, too. And in many cases they'll be right, and I think that's my point: being an only child is normal. It is one of several ways to get a normal, healthy, well-adjusted child. It's also one of several ways to get a sick little psychopath. One is as reasonable a number of kids in a family as zero, two, three, four, five. If you have parents who have a number of children for which they are ill-suited, things will probably go worse than if the parents have a number of children well-suited to their temperaments. The failure modes are different for different family sizes, but good and loving parents make good and loving homes for their kids in whatever numbers work for them.