Excluding Game Jams, I always work alone since game development is just a hobby for me and I have a lot of other work to do. So it's the best way for me!
The best part about working as solo is it gives you all the freedom with time restrictions, your projects design and development.
However, I think working alone works best for smaller projects, since it’s a quicker way to learn new game types, tools and hardware. It also familiarizes yourself how much workload you can handle for a single project.
Even if you are working alone, have other people around who are interested about your project to motivate yourself. Otherwise development can get lonely.