"The man who goes alone can start today; but he who travels with another must wait till that other is ready." - Henry David Thoreau
I solo because I like it. For me, there is no substitute for getting out of town, away from everyone, and traveling at my own pace. Most of my outings have been solo, and I've found that I can live comfortably with myself without the company of people, music, television, etc. I think everyone should experience a few days completely by themselves. Its very, very good - to me anyway. When there's nobody there but me and the mountains, I can shed the daily routines and really sort myself out down to base level. I've had many exciting adventures and some important realizations about myself and what I'm capable of that I would not have had if I had not gone solo.
The experience of a solo trip is completely different from the experience with a group. Not necessarily better or worse, just different. There is always a short transition period as I move from one reality to another. Usually, I find myself feeling a little lonely and out of place on the first night of a solo trip, but by the second night I don't mind it at all. I remember spending about 70% of my time on the first day telling myself all of the reasons that I really didn't want to be there. I heard some thunder and decided that the weather would be no good. My pack was too heavy. I might get hurt. On the second day I begin to break free - maybe spending 20% of the time thinking about how nice it would be to head home a bit early. After three days or so, I find my mind quiets down and I tend to act without a lot of self-talk. I don't concentrate on relaxing; it just comes.
How often does one get to spend a few hours, much less a few weeks responsible only for oneself? Time spent this way is a real treasure. Perhaps most people, like the tourist in Edward Abbey's "Desert Solitaire", can't believe that one can treasure one's own company the most. This may be a social/tribal reflex, bred into us both culturally and genetically. Adaptable generalists by nature, human beings survive and thrive better communally, rather than solo. As we start out to do anything alone, we have a nagging primal concern, even a fear, similar to what most of us feel as we hike into twilight until it gets really dark, that since we are alone, we must be doing something unusually dangerous.
The three words, "solo", "lonely" and "bored" have no inherent relationship. I have yet to take a solo trip and not meet some of the most interesting people. Had I been traveling with someone, I would probably never have spoken to them, and while they might have spoken to one person, they might not have spoken to two. Not having someone else along means I see and hear a lot that other people just dream about. Some of the loneliest people on earth live in cities surrounded by thousands of people. Anyone who's bored has no one to blame but himself. Bored people often think that someone else has the responsibility for providing the entertainment/companionship/etc. in life. As I was growing up, my parents did a good job at finding the balance between nurturing and supervision and taught my siblings and me how to do things for ourselves.
In "Mountaincraft and Leadership", Eric Langmuir lists all the reasons why solo is a bad idea, and then goes on to say that it's well worth ignoring them all! Among other things, he suggests that, for many, solitude, even if only occasional, is a basic human need and that a solo trip can be extremely rewarding. However, I want to warn people before they set out alone: if you want to stay safe, stay home. If you've never gone out alone, I suggest trying it for a two to three-day trip, and only after you have some experience hiking and camping in that area. Three days will give you time to adjust to being alone, and the previous experience will help guard against discouraging problems. Of course, solo travelers should also leave an approximate itinerary and contact information with at least two responsible people who can call out the dogs if you don't show up when you're supposed to.
Remember to be aware of your mortality; life is short. Remember you or those you love might not be around tomorrow, so enjoy while you can. Remember that interesting people are interested, and if you're bored you're probably boring.
Oh, and expect to have at least a day's transition time when you return to the "real" world - the displacement effect works in both directions (something I think Colin Fletcher mentions).