I rarely ride alone, simply because it's fairly boring, unless I'm trying to figure out a new and more challenging trail for me and my buddies, then I ride alone to scope it out (not the smartest thing I guess). Riding with someone is good because it makes you competitive in the way that you want to be as fast or faster than whoever you're riding with, and that way you gain strength and become a better rider. I mostly ride XC, with a few crazy detours here and there, I would definitely advise to ride with friends.
Ciao