I am torn when talking about this topic. The reason is I have lots of friends, some who run, others who don't, some who leave near me, and of course many in different parts of the city, or out of town. Whle I love to be with my friends, I do not enjoy running with them. The one exception is my wife, a long time competitive runner with whom I enjoy running. We often play a game where she handicaps me and we have fun competing against each other.
When I think about this topic, I believe the reason I do not enjoy running with friends is the logistics behind the run. Coordinating times to gather, deciding on the pace, agreeing on where to run, and for how long. The advantage of running alone is independence...when, where, duration, pace...all decisions I can make on my own and change during the run if I want. While this attitude may seem selfish, I rationalize the approach based on me doing what I need to do to enjoy my exercise. I believe long term commitment to an activity should be tied to its enjoyment. My maximum enjoyment from running is to run on my own.
If friends ask me to run, I will certainly not say no and more often than not, enjoy the experience as we have an opportunity to catch up and usually run faster or longer than I would otherwise...which is mostly a good thing. However, my preference is running on my own...even though my two closest next door neighbours and friends are runners of similar ability to me.