Although I like traveling with friends and loved ones, I've always found solo traveling to be equally fun. My work allows a lot of free time off, and many times my friends and family aren't able to match up vacation schedules with me. Also, when I'm by myself, I have the option of doing whatever I want from vegging out at the hotel to eating whatever and whenever I want… this is doubly important as I tend to be somewhat of an adventurous eater, whereas past girlfriends and traveling partners have usually erred on the side of extreme culinary caution. Still, sometimes it gets lonely on the road, and I've found some interesting places to meet people while on my travels:
– Hit up your networks before you leave: During my university days, I was lucky enough to meet a lot of international students. Through my friendships with them, I was able to crash at their houses in such places as Melbourne, Singapore and Seoul and meet a lot of locals through their networks. Also, by dint of being international students, they were much more likely to have friends in other countries as well. Accordingly, I was able to get introduced to people in several other countries around the world, and some of these friends-of-friends really went out of their way to play tour guide. You might be saying to yourself, "I don't have any international friends", but almost everyone I know seems to know someone or has a buddy in London, Tokyo or in other far-flung places. Although I have some misgivings about social media, Facebook is great for helping out with this.
– Couchsurfing.org: Being somewhat of a hotel-snob, you would think that a site dedicated to sleeping at stranger's homes would the last place I would recommend. You're right: I've never slept at a member's place or hosted anyone at my own. However, I've found that the forums on this site are invaluable in finding like-minded travelers who are open to meeting people with similar interests. Members tend to put up a lot of meet-and-greet events on the forum as well all over the world. I started on this site since 2007, and I still keep in touch with quite a few of them around the world.
Of course, there are lots of other tips out there on how to meet people as a solo traveler. These suggestions range anywhere from "smile a lot" to "stay at hostels", but I've consistently found fun times and a surprising number of lasting friendships using these two tips. Good luck!