If you ask parents they almost universally say they're much happier being parents, that children are the best thing that happened to them, and so forth, but there's clearly some reporting bias here. At least among my friends, the ones who understand retrospective and sunk cost biases and base rate neglect are the least gushing about the wonders of parenthood...
Most research seems to suggest that parents are actually less happy than non-parents, though perhaps different kinds of parents fare differently. The effects seem relatively small, in any event, and my suspicion is that in most cases people's expectations simply readjust to parenthood, as to other dramatic life changes (consider the famous comparison of lottery winners and paraplegics). And of course there's a lot more to life than happiness.
When I was 18 I decided that since children got more interesting as they grew up, the obvious thing to do was to adopt a 20 year old and skip the less interesting years. Or, going back to thinking about the other extreme of r/K-selection, one could imagine simply dumping millions of gametes into the ocean and twenty years later having a child knock on the door and say "Hi Camilla! Hi Danny! I'm yours", though it's hard to see how anything like that could work for a social species like H. sapiens.
I don't know what the 18-year-old Danny's criteria for "interesting" were, but I daresay you've moved beyond them by now. As a mother of two and grandmother of two, although I can't say I'm any happier than any non-mother/-grandmother, I do feel it s a privilege to be part of a human being's development into that "interesting" 20-year-old. Newborns are endlessly fascinating as they change and progress every day and usually you can only observe that progress if you are a parent. As the child grows, the acquisition of skills and knowledge and the discovery and nurture of talents are a source of delight only to those who have the opportunity to see them in the daily increments in which they take place. Of course, like all experiences, there are down sides as well as up. But the highs of being a parent can't be obtained any other way.
I talked to a few female colleagues, they said they felt being a complete woman on attaining motherhood. It is a feeling only a woman is privy to. Once they had that first child, there is no stopping for a second and more to follow if desired.
For me, it was when my first child started university that made me feel really special. Now, I am happy to introduce a wonderful young man as my future son-in-law, something to look forward to again.
Diyan, I wasn't nearly wise enough to understand any of that when I was 18! I was clever enough that the whole idea was intended as a reductio ad absurdum, though.
Once upon a time and place it might have made sense to adopt a 20 year old, back when dynastic inheritance mattered, now it's clearly a silly idea.
Kieran Healy at Crooked Timber makes an argument for incommensurability in What you can't expect when you're expecting.