I've been finding myself more and more dissatisfied with my work as I get older and regretting that I didn't do something crazy when I was younger instead. Its not too late for me to do some of those things luckily, but its very difficult to drop everything to do so (and its not clear that, for example, a years travelling will make me any less dissatisfied with my work, although at least it would help me refocus my life).
I think this is the crux of the matter. I know a few people who spent their 20s traveling abroad and having great life experiences, and now that they're in their 30s, they're like "Shit, I have no career, I'm working at Whole Foods / living off my parents / flat broke and I'll never be able to afford the life milestones that I took for granted like getting married, having kids, or owning a house."
I think the root of the problem is that evolutionarily, our happiness emotion didn't evolve to make us happy. Rather, it evolved to force us to take actions which increase our chance of passing on our genes. There's pretty ample evidence for this (Google [hedonic treadmill], for example), and yet we keep wishing we were happier. I guess we're fated to do this, because people who didn't wish they were happier wouldn't be driven to take actions to pass on their genes.
FWIW, I don't regret working hard (at some startups, and also one big corp) during my 20s at all. It was what I wanted to do at the time, and it's given me some cushion to do other things I want to do. Plus I got to work on some really cool projects with some great other people, and had a lot of fun doing so.
I've got a cool job as a software developer, and it pays well. Flexible hours. Good colleagues. But I don't give a flying fuck about the software that I'm paid to develop. I'd much rather be working on open source software.
Why don't I do it in my free time? Well I'm still studying part time, so I can't really. But this is my last semester, and maybe things will be better once I finish my studies.
It really sucks that time is limited, and you can never get it back.