Go all in
I love side projects. Google, Facebook, Twitter, and many other great companies started as side projects.
But that’s just the thing. They didn’t remain side projects for long. It’s hard to change the world if what you care about remains a side project forever. At some point it has to become the thing you think about in the shower, the thing that keeps you up at night. 9 to 5 isn't enough -- it has to become all-consuming.
Side projects can be a good way to de-risk what you think you might want to work on; but unless you go all in, you will not be able to experiment enough -- and fail enough -- to determine what is actually worth pursuing.
The company I work for now was initially called Dwelling. When Dwelling didn’t work, the founders had another idea and changed the name to ProPhone. When ProPhone didn’t work, the founders had another idea and changed the name to Topline Pro. Things are finally working! But it took 4 years. Nick and Shannon needed the time + energy that dropping out of business school afforded them, to figure out how to make the company work.
One analogy is relationships. Imagine if you treated your boyfriend or girlfriend like a side project. That would be a disaster! You would never reach the level of depth required to sustain anything of substance. My belief is that the second you’re truly convinced you want to spend the rest of your life with a person, you should fully commit. Maybe it will turn out to have been the wrong person; maybe you will live and learn and change your mind; but as Lord Tennyson once said, "Tis better to have loved and lost than never to have loved at all."
Side projects are great - but the meat of life, the stuff that makes life worth living and that you'll one day reminisce on with grandkids - those things only happen when you go all in.
Lives aren't defined by side projects. They're defined by the things one goes all in on.