The reason to train in a group: you'll perform better

Besides that it is fun to train together; is there any other excuse you need? Science is giving you a very good one: you will perform better!

I usually train alone. It is less planning. Just put on your gear and go. I find that when I do run together with someone: it feels easier, less far, faster. I thought it had to do with the distraction the other person offers when you talk a little.
Another reason might be that you are competitive and do not want to give in towards the other person.
non of the above; It is a simple biochemical process at work!

During excercising you body produces endorphin. A hormone well known for the runners high it can induce. Like morphine it helps to deal with pain and that is why mother nature gave us this endorphin.

Research from the University of Oxford shows that athletes who completed a training on their own can stand less pain than athletes who trained in a group. This is an indication that those who trained in a group had higher levels of endorphin in their blood (Source: BBC News 2009 & site Oxford University).

Besides the positive effect on sport performance they think is might also explain why people feel euphoric when celebrating a mass or making music together.

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Could it be that this evolved when we were hunting together? Read my posting on "Born to Run". The author explains how we might have evolved the way we have because we were successful in chasing animals to their deaths thanks to our body which is build (or should I say evolved) for endurance running.
This will work when you collaborate as a group. and when do you want to work together? When it is fun and when it delivers a result. The fun part comes from the endorphin. The result from the food, the meat from the animal you chased to death with your companions.

OK, you no longer run together to get the antelope but something you body developed over a long period will still function as is did when we as a species were still out their running our food to their death. Maybe Christopher McDougal should rename his book: "Born to Run Together".