If this sounds lonely to you, fear not - solo travel doesn't necessarily mean you'll be completely alone the whole time. I've found locals are more likely to strike up a conversation if you're by yourself. You might even run into a friend - on my recent trip to Japan, it turned out one of my friends was there at the same time - we had a fun day shopping and going to the Ueno Zoo to see pandas. Another friend who lives in Tokyo took me to a baseball game and a rugby game and gave me tips on the easiest places to order food as a non-Japanese speaker.
And by a bizarre coincidence, I ran into my old barber and his girlfriend at the Cup Noodle Museum, of all places. This turned into a crazy night out drinking in Shinjuku. If I'd been with a travelling companion, I'm not sure I would have sought the company of others quite as readily.
One of the downsides of solo travel can be the added expense of not having someone to share a room with - though I would rather pay more and avoid this. Sometimes I snore, which means my roommate doesn't get a good sleep and I'm left feeling guilty about it for half the day.
And that takes me to my next point - anyone who has travelled with a good friend, parent or partner ends up getting shitty at them at some point.
Some aspects of travel make me irritable - the jetlag, the tiredness, the culture shock - but if I don't have anyone around to be annoyed with, there's nothing to do with that feeling and it quickly passes. There's a reason so many couples break up after going on holiday together.
Overall, there's nothing like being by yourself in a foreign country - having to solve by yourself the problems that might arise is a great way to get out of your comfort zone. It's extremely humbling and I can guarantee that you'll grow as a person and learn plenty about yourself in the process.
As Sartre said: If you are lonely when you are alone, you are in bad company.