A healthy and fit human is a social animal. We survived our evolution for a hundred millennia by working and cooperating in small groups: the family and tribe, and particularly the hunting party--the original cross-country team. If your cross-country team consists of the Olympic 5000 meter champion and six C-shaped joggers (people whose hips and butts hang out behind them instead of aligning vertically with their centers of gravity), it will lose every meet! The scoring of cross-country is based on the recognition that at its roots, this is a team endeavor. Since humans could not have successfully hunted mammoths as lone heroes, they had to chase down their prey in packs, the way wolves do. So, it's in our genes to run in groups. And most long-distance runners do at least some of their training (as well as all of their racing, of course) in groups. Lone heroes have been romanticized in our pop-cultural consciousness by solitary comic-book superheroes, cowboy heroes (a generation ago) like the Lone Ranger and subsequent John Wayne characters, and action-movie characters who are on the run from their erstwhile-colleagues at the CIA and have to survive by their wits. But the biological reality is that humans are always interdependent. (Even the lone, Montana-cabin recluse with his bag of potato chips, six-pack, and shotgun can't survive without there being farmers out there somewhere growing potatoes and hops, and truck drivers hauling the potatoes and hops to the processing plants, and the entrepreneurial-immigrant guy in the local 7-11 selling the chips and the beer. Total independence is a delusion.)
On the other hand, in our long evolution, cooperation was essential but limited by nature. The hunting party provided mutual protection, but if one member sat down in a funk and refused to continue, he probably got left behind and eaten by a lion. His funk genes weren't perpetuated. And in the world we have inherited, society functions best if we cooperate but also continue to carry our own weight. We are interdependent but also independent.
For the ultrarunner, to keep that sense of independence strong, it's helpful to do a significant amount of running alone. If you can run with a companion or group once or twice a week, that's good. But chances are, you spend most of your time both at work and at home interacting with others, so you probably don't lack for social experience. What you may not have so much of is true independence, to the extent that that is possible for a fundamentally social animal. A few days a week of solitary running can do wonders for that. To practice feeling independent and self-sufficient on the trail is not just a boon to your running; it is one of the great rewards. Take your water bottle, but leave the smartphone at home. At least part of the time, it's important to connect with the air, forest, wildlife, and the signals emanating from your own body, not just to chat with companions.