Eavesdropping is a bad, bad thing. You really shouldn’t do it, because it’s absolutely none of your business.
But think of all the interesting conversations you’re missing out on? I could never let that happen to me, because one of the things that always make me smile is eavesdropping on other people’s conversations in the bus, behind me during lectures, at parties, whenever.
Sure, you pick up on things you might not want to hear, but the trick is to listen to people you don’t know and that don’t know you. Sometimes you strike gold: