I guess I never got the rule book.
Over the years I have had a number of great conversations on planes. If one is not in the mood, it is easy to be polite at the beginning of a flight but then immerse oneself in a computer, book or music. But, if a nice conversation is welcome and available AND THE OTHER PERSON IS RECEPTIVE, why not?
I probably have a nice conversation once a month or so. Last month, on ORD-LGA, I talked with a German who was president of a division of a multi-national CPG company. We had some common experiences, shared some restaurants in NYC and Chicago and had a thoroughly enjoyable conversation.
I do think that PED use throughout the flight will drop the occurrence of these types of conversations as people immerse themselves immediately upon boarding and only emerge on landing.