According to some research out of Princeton University, running by yourself has few brain benefits. According to Dr. Elizabeth Gould,

In the absence of social interaction, a normally beneficial experience can exert a potentially deleterious influence on the brain.

The problem comes from the generation of a stress hormone called corticosterone, which reduces the creation of new brain cells. Running in a group negates those effects and more effectively boosts communication between neurons and spatial awareness.

The studies were done with rats, but I am not going to buy this completely until there has been more research done on it and there have been tests with humans. I have often gotten through creative blocks when I took some time off to go for a run, and I very much enjoy a good solitary run on a fairly regular basis. Of course, I also enjoy running with others on a fairly regular basis as well.

The article uses this research as a reasoning for why it is more difficult to work out by themselves than with other people, but even if everything in this study directly translates to humans I can not see the link between the hormone and not wanting to run. People are inherantly lazy, and without some sort of pressure to do a workout it is very easy to find an excuse not to do it. Social pressure is one of the best ways to ensure that you follow a workout plan.