Many of us feel the need to be overly cordial with old coworkers we encounter in the elevator, and it's usually out of guilt. It's harsh to say, but it's doubtful we'd even notice if they croaked over unless we read it in the obituaries. It's not necessary to guilt yourself into pointless small talk. Just smile and say hello.
Rudeness and bad manners - and books about them - are on the rise. Technology clearly has much to do with this. Even "smart" technology has made many of us seem "less smart." (That's a polite way of putting it.) It's certainly made people more distracted and self-centered, both of which breed bad manners.