Something in us innately finds line-cutters aggravating. It is a behavior that makes them seem to act outside the very basic level of decency and respect we expect from each other in public spaces. When someone pushes in front of others who have been waiting patiently for an awfully long time already, it is not about minor inconvenience, it is a sign for an overall disrespecting of the order and fairness we count on in regard to the collectivity.
You're standing in line at a busy coffee shop, anxious for your morning brew, when someone casually walks right on past everybody and proceeds to stand at the counter like they have been there all the time. That isn't being a pain no, I think that's just plain rude. It's like the person is trying to tell me that their time is more valuable than mine.
When one person cuts in the line, it is not just about them getting their coffee or grocery a few minutes earlier, it disrupts the whole process and creates absolutely unnecessary tension amongst everybody that's been patiently waiting. What is the point of the queue if people are not going to respect it? There goes the system, broken and we have a big mess of frustrated people in a ridiculously long queue.
When I see someone cutting into the line, it is hard not to feel the surge of anger and not to give in to saying something. But the confrontation of a line-cutter sometimes turns into an awkward confrontation and occasionally escalated into arguments. Line cutters are mostly rude so confronting them can easily lead to a serious conflict.
Either bottlenecking your frustration or risking an open dispute, it is a lose-lose situation almost always.
I think the real issue at hand is an empathy deficit. Most of us recognize that waiting for our turn is just a simple respect that keeps society in smooth operations. However, to the line-cutter, it is "me first," period. They do that without a second thought to how yheir actions are going to affect others. It is a very selfish mindset, one that undermines the simple yet essential social contract on which we count for our daily lives to run as usual.