A line is an involuntary combination of people who are simultaneously irritated with one another and focused on a single, common circle of interests and goals. This leads to a mixture of rivalry, hostility, and collective sentiment, a constant readiness to close ranks against a common enemy - anyone who breaks the rules.
Sometimes we do that to ourselves -- we pit our desires against one another. We insist unnecessarily on seeing one aspect of our personality as being at odds with the rest of ourselves.