I'm an only child, and have never regretted it. I don't think I turned out too spoiled or selfish, and it's been quite a set of discoveries to have two daughters of my own. Their sibling rivalries and synergies awaken no deep-set memories for me—they are as new for me as for the girls.
I have noticed one quirk, not unique to only children but probably more prevalent in us: I genuinely enjoy being alone. Don't mistake that for not enjoying being with other people. My wife is my best friend (not to mention a babe), so it's great to hang around with her, to go out, to snuggle. I like playing with and talking with my kids, and going for dinner with my parents or other members of our families. I'm sociable, gregarious, and sometimes you can't shut me up in a crowd. I have some of the best times of my life laughing onstage and off with the guys in my band.
But often, at work, when people ask if I'd like to come for lunch, I say no, and take the time to eat and walk and read and listen to my iPod by myself. In summer I commute by bicycle, and would rather ride that road alone than with a buddy; in winter I take the train, where I can be alone in a crowd like almost everyone else. When I'm on a business trip or a vacation or playing a gig, I'll often take some time away from my family or colleagues to roam around or have a meal or just a few hours on my own, because for me it's fun. I'm not morose in any of those places, longing for friends.
Sure, I'd rather my wife were there most of the time (frequently she is), but I can even sleep in a strange hotel on my own without trouble when necessary. And at home some of my favourite times are these ones, late at night when everyone else is asleep, and I can geek out on the Internet or with a guitar or in front of the Nintendo, or take a bath.
Maybe you don't enjoy times like that as much as I do. That's okay. But I don't think it's all too strange that I do—we all want to be alone, sometimes.