How Much In-Flight Chit-Chat is Too Much?

Airline etiquette is a delicate, complex issue, as we saw in our discussion about whether or not it's good manners to recline one's seat. Well, here's another in-flight conundrum: How much chatter is too much?
In asking the question, I'm reminded of a flight I took a few years back from Athens to Boston. It had been a grueling day of flying that began at 5 a.m. on Santorini, and I really wanted to sleep on the long transatlantic flight home. Unfortunately, I was seated amongst roughly two dozen older Greek folks, all of whom appeared to know each other and spent the entire flight in animated conversation. No sleep for me.
So, readers, where do you draw the line? Because for each person looking to sleep on a flight, there's likely a group looking to talk. Who's right? Should we feel entitled to have a boisterous conversation with our seatmate? Should we keep the chit-chat to a minimum, and use our library voices when talking becomes necessary? Is it OK to talk to strangers seated next to you, or should you assume people want to be left alone? Leave a comment below with your thoughts. Thanks!