I don't want children and never have. There's still time, biologically speaking, but the only way I could realistically see that changing would be if I met someone I really loved and specifically wanted to have children with them, rather than in the abstract, if you see what I mean. I accept it's a possibility, but I've never met anyone like that so far and I'm not actively looking for a relationship, so it doesn't seem very likely.
I've known since I was a teenager that I was far more interested in maintaining strong friendships than having a family or a romantic relationship, and actually that worried me a huge amount at the time. I had visions of all my friends getting married and having babies when we were in our late 20s, and thought I'd be lonely and left behind as they focused more on their families. In reality, of course, that's not what happened at all. A lot of my friends are single and/or childfree themselves, the ones who did have children didn't all have them at the same time, some of my best friends are 10+ years older than me (inconceivable when you're 16!) and their children are nearly grown, and as I've got older I've simply found my own interests and felt less need to be around people 24/7 anyway. I also wasted a lot of my twenties making myself miserable by chasing relationships that weren't good for me and that I didn't really want, just to try and feel normal, and though I'm very happily single now I still feel the loss of those years that I'll never get back.
I think this experience taught me two important things that helped me clarify my feelings around not wanting children. First. that it's not worth doing something you know isn't right for you just because other people insist that you must want it really. And second, that it's not worth making decisions based on hypothetical worries that are decades away from materialising, because by the time they come around both you and your situation will be unrecognisably different from whatever you're imagining anyway.
Relationships are to a large extent what you make of them, and provided you're open to other people and willing to make an effort it's always possible to form new bonds even once you're older. My parents uprooted their entire lives in their mid-60s to move to a city where they didn't know a soul, got heavily involved in community projects and hobby groups, and had better social lives there than I did in my twenties. We get along well, but I live on the other side of the country and only see them a few times a year - in terms of loneliness, they get far less social contact from me than from their friends and neighbours, some of whom are younger than me and in very little danger of dropping dead from old age any time soon!
And of course, many people are lonely through no fault of their own, whether due to social anxiety or ill health or simply bad luck, but those things can all affect people who do have children as well. My maternal grandfather had five children who loved him, but it didn't stop him being lonely at the end of his life when most of them lived hundreds of miles away and he had few local friends. Even if you have a loving family, if you're entirely reliant on them as your social network, how are you going to cope if, for example, your children have problems of their own that leave them with no extra capacity to support you as well? With the exception of those who thrive in solitude, everyone needs strong social connections with a variety of people - unless you're extremely close and they live nearby, your children by themselves are unlikely to be enough.
And as others have said, you can't even guarantee that you will have a perfect loving family. I used to be a secondary teacher and have spent years working in schools in some capacity, and it's easy to make a flippant remark about how I've known enough teenagers in my life to put me off having one of my own. Truthfully, though, I do like them - otherwise I wouldn't still be doing it - but I've seen so many variations on what can go wrong, from behaviour problems to bullying to addiction to abuse to serious mental or physical illness to, tragically, suicide and murder. As a teacher I never learned to detach enough, and every single day I felt like I was failing kids by trying to explain the structure of the atom instead of being able to give them the kind of support they really needed. I can't even begin to imagine how much worse that anxiety and guilt would be going through the same issues as a parent.
But mostly, for me, it's that there's already so much I want to do with my life, and I already know I won't have a chance to do it all before I die - I just don't have a spare few years to spend on sleep deprivation and changing nappies, let alone the next fifteen before they move out! Frankly, I'm not even sure I'll ever find the time to get married, never mind raising a child 
I hear what people say about the depth of love they feel for their children and how it's not possible to experience that until you have a child of your own, and perhaps they're right. But equally, I doubt most people have experienced the depth of awe and wonder I feel when I study particle physics, and that's definitely not for everyone either. No-one gets to experience every single thing in this world, that's literally impossible. But for most of us, hopefully, there's still a vast, varied and exciting selection to choose from, whether we end up having children or not.