When you are unable to share a headphone with your friend because a band produces their music with complex stereo harmonies and effects, and one requires both headphones for the music to make sense.
i.e. there are guitars on one side and vocals on the other, so taking one headphone would mean hearing just drums and a guy singing
"Dude can i listen to your iPod too?" *Sigh* "I guess... but they are an antisocial band, you need both headphones. here take the whole thing, it won't be the same if you don't"
Hot girl walks up: "heeeey lets listen to music together!! <3" you: "i cant sorry it wouldnt make sense to you..." her: ? "ok whatever ill go hook up with tom, he has some lil wayne, not your weird two side shit"