Sidenote: I work as a product manager full-time at a company, but I also have my own app on the side. This gives me two different perspectives around building product, one for something already in orbit and the other getting something off the ground. This post is about the latter (my own project).
There's a million reasons why people start something. I can only speak for my own, but if I had to bet, the most common reason is they just can't help it. Not doing it isn't an option.
I started building Revere because I needed it for myself and there was no app like it. I also just wanted to build my own app: do all the design, choose all the animations, decide all features it would have (i.e. all the ones I wanted), etc. I didn't think any more deeply than that.
The more interesting question came up when things started to get difficult, and I had to ask myself "Why continue?" – that one's come up many times over the years.
In the early days of Revere, I wanted to build an app but I also thought it would be easy money; build an app in six months, launch it, and watch money roll in while I move on to the next project. So it started off as something I wanted for myself but I eventually saw there was much more going on.
I've often said Revere (only half jokingly) that Revere is a matter of life and death for me because it's forced me (and continues to force me) to face fears that were (and are) holding me back in many areas of my life. For example, the fear of putting myself out there is a big one. And at several crossroads, the only reason I kept going was because I knew that if I stopped, it would mean giving up on overcoming my fears and, effectively, my life.
So it turned out that "Why start?" wasn't the interesting question at all. It was "Why continue?" that helped me understand what was really behind that original need to make something.