I try to speak to my seatmate like 0%. I know that sounds churlish. You just never know. The person might be a chatty-catty. If you wear over-the-ear noise-canceling headphones, which you must if you're going to travel, then you have a built-in signaling mechanism for I am now ending the conversation. Just wait for a pause. By the way, I will also say this: I have learned a great deal over the years from talking to people I didn't know.
Are those travelers on their laptops just showing off? Why does V8 taste better at 35,000 feet? And why won't Angela chat with her seatmate?