It's the same feeling people get when they go to the DMV (similarly understaffed, similarly line-heavy and similarly useless most of the time). "I never need this building except for the few times every year when I do need it, and when I do, they spend every second screwing me," they think. You feel like some organization has gone out of its way to inconvenience you, specifically, and something in your brain temporarily snaps because of it.
At the DMV or the post office, unlike anywhere else, when someone enters and sees a line, they immediately don't believe it. They think, "No, no, this can't be, where's the line for -- I just want to mail this small box from this part of America to another part of America. Where's the section for that?" When they realize that the line is the line, they either leave (a stupid plan -- they still need to mail their stupid thing or stamp their license or whatever), or they accept the fact that they will wait on this line, longer than they'd ever anticipated. No one ever budgets their post office time correctly, everyone always assumes it'll be an in-and-out trip, even though it's never been an in-and-out trip, not once for anyone.
If you want to see hate on a person's face, go to your local post office and watch people. Watch them seethe with hatred while in line. Hear them audibly growl when one of the patrons in front of them forgets to fill out a form or asks to borrow a pen. I've been waiting in line for a movie or show or to get into a restaurant, and I've had pleasant conversations with strangers to pass the time, but never once has that happened in a post office. No one even offers an obligatory "This sure is a line, huh?" Everyone just stares straight ahead and uses their peripheral vision to scan for weaknesses in anyone who might be ahead of them in the line. Patience is gone, empathy is gone, logic is gone. All you care about is mailing your goddamned sweater to your goddamned aunt, and you are not eager to make friends in the process.