I have personally enjoyed travelling solo and with friends. They both have positives and negatives. As people have said at least you are guaranteed company with friends around, and there's much to be said for sharing experiences with someone who you know inside-out or at least well.
Having said that, travelling solo has always pushed me out of my comfort zone into seeking out company (if I was in the mood for conversing). Meeting like-minded people in new countries has always for me been a very enjoyable experience.
It's fair to say also that even being in close proximity with a good friend can lead to friction on occasion because that close proximity can be a very different experience to your usual relationship away from travelling. Perhaps I should select my friends better though!