Case 1: I started Fiat Lux with my friend Nick, who I met in college. We worked together on it for eight years. He was fully committed and put in just as much work as I did. Even when things were tough and not working well, I knew I could count on him, and he stayed to the end.
Case 2: For a superresolution radio app I recruited a small team of friends who I had worked alongside before for years. I knew they were all great people and very competent. For the first year or so it went well, but when we didn't get any momentum going, life got in the way and most of the team found other things more important. In the end I was the only one still working on it and we dissolved the partnership.
The differences between those cases, as I understand them:
1) Younger vs middle aged - family responsibilities get in the way
2) Prior interest - I think Nick would have worked on fusion if any opportunity had come his way, while my later team wasn't nearly as interested in superresolution as I was
3) Unequal previous time investment - when I brought Nick in, it was still pretty early in the project, so it was a journey together. But when I recruited the team, I already had a patent, basic code and years of work done, and I think this left them with less of a feeling of shared ownership.
4) Team dynamic - with the team, when one person was excited, it got everyone excited, but when one person was discouraged, it made everyone else discouraged. Managing this dynamic was difficult and distracted from the work. With just me and Nick things were simpler.