In the film Fight Club, the Narrator describes single-serving friends. Here's a helpful Yahoo Answer on the subject. I've discovered, while traveling, single-serving friends are everywhere, not just airplanes. Especially when you're traveling alone.
For example, my Hamilton experience was full of single-serving friends - the people I spent three hours with, waiting for the Ham4Ham; the Green Dress girl I fan-girled with over the "In This Performance" sign; the attendee I chatted with while stage-dooring. These are people I will never interact with again and yet for a few minutes, we bonded over the shared love of a musical.
Another place these are extremely prevalent: tour groups. While in Austria, I took a day trip to Eagle's Nest. I sat next to a trainee from the tour company; he was job shadowing the tour guide so that he could start doing the tour in a few weeks. We chatted the entire bus ride (two hours) and during our walk of Eagle's Nest. That's a lot of time to cover a lot of ground. We started with the obvious (Salzburg, his job) then veered off, covering television shows, books, other European cities we had both visited. Similarly, last year, in Rome, I did a half-day tour of the Colosseum and the ruins. There was one other American couple on the group and we stuck together during the tour. There was the usual - travel talk - and the unusual; they discovered I worked for a medical device company and most of the time we talked about Big Pharma and the future of various medical devices.
For some reason, outside of group tours, Paris has been the biggest single-serving friend place I've encountered. There was the Iowa native living in Paris I had lunch with at the Museum d'Orsay, the Miami couple I shared travel tips with on the train, the New York couple whom I shared Easter lunch with outside the Shakespeare and Company bookstore. The Chinese man I gave Paris suggestions to on a long subway ride. Like waiting for Hamilton, there's something bonding about travelers in a big city, an immediate and temporary connection.
Airbnb has made this even easier. While I was in New York, I stayed at the same Manhattan Airbnb for five nights and had the chattiest host - we talked every night when I got home, me telling about my adventures of the day, him giving me advice for the next day's plans and sharing stories about his time in NYC. And I'll probably never see or talk to him again.
Hostels are much more hit and miss. I've stayed at four - one in Amsterdam, one in Berlin, one in Brugges, and one in Salzburg. For the most part, my temporary roommates and I avoided each other. But Amsterdam, the first one I stayed in, was unique. A woman from Brazil shared it with me and every night we chatted, in part in English and German, sharing our travel time and gossiping about the hostel and our other roommates. An older American woman would come in much later and tell us what she had seen during the day (usually something scandalous in the Red Light District).
Additionally, my company hired a third-party to help me manage my move to Germany. I was assigned someone, Julia, to help me navigate the red tape. With her, I applied for a residency permit and a parking permit. She was with me when I got the keys to my apartment and helped me get a German bank. For the first month I was here, I spent several hours with her a week. And then the parking permit, the last thing she helped me with, was obtained and I'll probably never see her again, as someone else is liking to help me with my exit in December.
Single-serving friends: a reminder that the world is both very small (everyone on my Austria trip seemed to be from Missouri or Kansas, the American I sat next to in a crowded French restaurant, everyone in NYC loves Hamilton!) and also very large (all those connections, all those conversations, gone in a moment). It's been a joy of my time traveling in Europe and, to a much lesser extent, the States, to get to meet people from all over the world and have a glimpse into their perspective and experiences. My latest single-serving friends from my trip to Spain at the end of June? A British couple on the day of the Brexit vote, explaining to me why they were probably voting 'Leave,' over a lunch in Valencia and an Irish woman, sharing tips for my Spain trip and also upcoming trip to her hometown of London, over drinks while hiding from the rain in Barcelona.