Solo backpacking (i.e. in the backcountry, not campgrounds) is a very different experience than with friends. The first time I did a longer trip (6 days) solo, I was terrified for the first 24 hours, but then eased into it and ended up having a really good experience. It can be intense, though. Even after the fear passes, other strong emotions might come up. Overall I prefer going with friends but I'm glad I've done some long solo trips too.