I trained almost solely alone for the first 6 months I did parkour. Then for the next year or so, I was usually alone. Now the majority of my training is with one buddy (during college) but I still train alone a lot, especially during breaks from school.
I am more focused when I train alone and the accomplishment of something new is more satisfying. I have a better understanding of a technique when I learn it on my own. Overall I think you learn things more comprehensively and progress faster when you are alone.
I think its kindve sad when people wont do parkour just because they dont have anyone to do with. They are missing out.
However, I will admit that training with other people has some benefits as well.
i started parkouring about three months ago with two of my buddys and so I rarley ever practice by myself, but I love having them there to help because they are a lil better then me