I started noticing this trend when I started working out of college at a consulting firm. My co-workers were rarely without their headphones when sitting at their computer. I quickly realized they are used, partially to break the monotony of IT Audit, but also to display a look telling others “I’m far to busy to be interrupted with what you have to say, please leave me alone.” After trying it out, and then forgetting my headphones one day, I understood its need in a workplace that doesn't require much interaction with others. It is a solid way to avoid distractions and focus on what you have to get done. I listen to Pandora most days while working (currently listening to a Talib Kweli 'station') and have found the practice very beneficial.
1. When at a place of business trying to purchase something without expending the minimal energy to take out your headphones in order to hear the salesperson speak with them. Instead you would rather yell to compensate for your hearing defficiency and annoy those around you.
2. When on the subway, you feel the need because you must think those around must care enough to turn your music up to a volume that MUST be painful. This is the equivalent to those that roll the windows down in their car and play their music so loud while cruisin'. I must admit, I engaged in this practice when I was 16 and still occasionally when I am really feeling a song; however I don't make a practice of it to be noticed. There is a big difference.
3. People walking down the street, or anywhere for that matter, in a group talking to each other while listening to music (which I can sometimes also hear). Is the other person that boring? Are you playing the music so low it doesn’t matter anyway? Are your ears that developed to comprehend both stimulants that well? For those that will say it is easier to press pause or turn the volume down than take their earplugs out - it isn't! Also, your extreme lack of tact in the situation should overcome anything that may be perceived as too much effort.