Good point. I think it breaks down to accountability.
If you work with 10 people on a project, you are not as accountable because everyone else is responsible for the project as well.
However, if you are working with 1 other person, that person holds you accountable for everything; you have no excuses to not complete the project.
Finally, if you are working by yourself, you are only holding yourself accountable; this is bound to fail. You need some external influence of motivation to continue on, and oftentimes you need more than your own motivation.
Taking it to the extremes, if you work with 100 people to lear, then you probably won't have the motivation to do it. There are many reasons why, but to make a simple argument: the more people, the worse. However, if you work with just your friend, then you rely on one another to complete the goal. It is ment
I have generally believed that, too. But, on second thought, wouldn't having more people complicate planning thus thwarting progress?