What I found is:
> All I want is to make enough to cover my living costs and in my spare time work on side projects, code or tinker around. You know, do things that are actually fun to do and allow me to learn new stuff.
The side projects I'd like to do cost money, unfortunately. Not a fortune, necessarily, but enough that I need to be earning decently to afford to develop those side projects. And it is a fortune to build what I'd really like to. When I was much younger, pure software projects seemed enough. But ideas have a tendancy to grow :-)
Then there's time. As I got older, I realised the time I was putting into side projects was either not enough to build what I hoped to build, so slow that it become socially irrelevant by the time I did anything (others, paid, always had more time to put in), or it was seriously exhausting doing it at the same time as a job.
Over the years this built up into increasing dissatisfaction. When I was younger I had dreams and thought I'd gradually get around to some of them. As years passed by, I realised it's not going to happen for the bigger ones, and I'm sad, already mourning the loss of potential.
On top of all that, life adds major random events, which take major random time and money and all your personal energy for a few years. I don't think that's limited to younger or older people especially, but the longer you live, the more likely it is that major things happen at some time in your life, either to you or people you care about, which take over for a while.
I couldn't care less about profit or money personally, for its own sake. But I have dreams about things I want to make that are much more than just toy projects, and I want basic personal security. It turns out these aren't readily available after all, as I assumed when younger.
> If I have an idea i'm very passionate about I'd like to just share it with others because they might find it interesting
That's a reason to be careful who you work for.
You often can't freely share your most passionate ideas if your employer has a claim over them. And if you're working in a different field, to ensure that kind of claim doesn't hold water, limiting the time spend on a passionate idea to outside-of-work time is quite restrictive too.
All this is not necessarily an argument for founding a VC-backed startup, of course. But it's why just working for some company or other for a whole working life doesn't necessarily work if you have big ideas you're passionate about using.
ps. I wrote this as a contrast to the comments about how relationships, family, marriage, mortgage etc. will matter at some point. I think that comes over as a bit "you don't know now but one day you will have feelings for someone and they will expect you to spend money on them" I wanted to show something different, that happens even if you find a partner who is frugal like yourself, and doesn't cost anything :-)