@rosano I think it depends on your goal for a project.
If it is to accomplish something, and you're sure working with other people will make it better; you should involve others.
If your goal is to create a "lifestyle business" (nothing wrong with that), then it's fine to keep working alone. But you have to assume the limitations of that model.
It's not only one or the other, but I find it very difficult to believe that you could fully achieve both. You should choose one and embrace it.