I've gotten the most enjoyment out of side projects when I know that I'm making them for close friends first, and any extra popularity is just a bonus.
I have also found that building things "for fun" and "for the sake of learning" has been both fun and educational.
That is a really good idea: make something for someone you really care about and who really cares about you.
I have built a side project with a user base of less than ten people. Mostly people I have known for years. The only counterargument is "why aren't you using <more popular software> instead?"