I have tried and failed to write coherently and this is a long mess, sorry. Just some of my rambling thoughts on this as it's touched a few nerves.
I think, as some others have said, it depends on the person, their situation, and also society's attitudes. Someone who doesn't want children and who is financially secure will I presume be happy. But it can be very difficult surviving, financially, as a single person.
I've always wanted children, it hasn't happened, and it's incredibly painful. I don't know how much of that pain is caused by feeling acutely aware that much of society sees childless women as worthless, or at least a bit of a "failure" and not very important to society. I know not everyone feels this way but it is a prevailing attitude in media, government policies, general public views.
I've had a strong maternal urge since I was very young but perhaps I would find things slightly less painful if there was a better attitude in society. I don't know.
It's fine being single and childless until things wrong. I know, as a DV survivor, being childless has made leaving very hard in terms of acessing longterm housing in particular, but simply that I feel like an outsider as many DV organisations highlight their help for women "and their children". Funding cuts have meant many DV services are more like child protection agencies. Of course children are a priority, it's just a shame that women aren't seen as deserving of help simply as women. Women are seen as mothers only and not women, worthy humans, in their own right.
I know I'm more sensitive because I so very much wanted children and also because my situation is not so great, but I do think this sort of attitude towards women, that they aren't "normal" for not having children contributes towards people going for and staying in unhappy family setups, why some people who don't want children have them - because of society attitudes. My mother never wanted marriage or children and felt pressured by family and society expectations.
I also wanted marriage, or at least a stable loving adult companion, but as I've got older (and experienced a long abusive relationship) I've reassessed my feelings and I've realised that I would be happy on my own, but only if there was a safety net....that doesn't (any longer, if ever?) exist. I suppose some places and people have that, a good family, friends, and neighbour support network, but often when you are alone and unwell or in some other bother, it's only your partner or spouse who will/can take you to hospital, do your shopping, make important calls, and all the other boring or unpleasant little chore like necessities.
If you aren't financially secure and in good health, it's hard to survive single and childless. I'm stuck with an abusive partner because I won't be able to access long-term housing without him. It's one of the reasons I semi-consciously overlooked red flags at the start, that and my biological clock ticking.