Firstly, please don't judge me for this. I don't deliberately listen into other conversations. I'm naturally inquisitive, it's true, but I am not one for eavesdropping into a private conversation in the same way I wouldn't want anyone to eavesdrop on my conversations. However, on a rush hour of a morning, personal space is in short supply and if somebody is having a conversation right next to you, it's impossible not to hear it.
Of course, hearing it isn't the same as listening. But even when not consciously listening to a conversation, the brain can still pick out the odd word, and sometimes there are words, or phrases, or threads in the conversation which act like a beacon to the conscious brain - drawing attention from your book or current "Words with Friends" game and pulling it into the conversation of the person next to you.
I was on the tube yesterday morning and this happened to me. I was trying to finish off "The Woman In Black" and I overheard the conversation of the woman next to me, and the guy who was standing next to her.
Sometimes, I worry think about the ultimate futility of my job. Don't get me wrong - I enjoy my job, and I believe I make a good contribution to the company and the team - but I don't save anyone's life or make the world a better place at any fundamental level. But when you overhear a conversation of someone describing how their task for the past week has been to organise the re-upholstering of the office chairs into the new company colours - suddenly life as a software manager doesn't seem quite so bad.