Getting out and causing a ruckus with a bunch of friends is a lot of fun. I always enjoy showing people the roads I like to ride, and it's nice to have someone to chat with at the stoplights on boring roads. At the end of the road, a lot of times there's a restaurant or something else that's best enjoyed with friends. Also, it's nice to know that you've got a buddy or two along in case of a breakdown or some other unpleasant event.
On the other hand, riding alone can be even more fun. For one thing, when you're riding alone, you can ride at your own pace. Some days, I like to ride at 90, other days 70 is just fine. Trying to keep up with someone else when I'm having a slow day sucks, and waiting for someone when I'm having a fast day also sucks.
In addition, a lone biker meets a lot more people along the way than a group does. Other people are simply less likely to approach a group of bikers than a solo rider. Chances are you'll meet a variety of interesting folks while travelling alone that you wouldn't even see if you were with your buddies.
An additional problem with travelling in a group is that motorcycle to motorcycle communication can be rather difficult. So if you spot something interesting along the road, it's likely you won't get to stop and check it out if you're in a group (unless you're the lead rider).
Riding in a group is a good time now and then, but riding alone opens up all kinds of opportunities for adventure. Besides, did you really start riding because you're a team player? Get out there and be a rugged individualist - by yourself.