You won't like hearing (reading) this, but take it from a complete stranger: hearing loss is a real, um, pain. Don't risk your ears, the volume limit is there for a reason. You're going to need your hearing a lot more in your later years than you do now, so take some precautions.
As for how it got set, apparently that iPod's not really yours if you didn't the volume limit, because somebody had to have done this.