system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do people get annoyed? What’s triggered in the brain to feel irritated and why do we have pet peeves?
gpt
Annoyance is essentially a stress response in your brain and body, which causes the release of hormones and neurotransmitters to help you respond to that stress.

Some annoyances cause this reaction because they make things more difficult for you (annoyances like being made late, being hindered toward a goal, anything that means you have to do more work, do something differently than how you were planning to do it, do something faster than you wanted to, skip doing something etc). 

Some annoyances cause this reaction because they cause something called "cognitive dissonance." This is basically when your brain is getting input that is different from what it knows, which it does not like, as a rule. This would be annoyances like someone acting hypocritical (doing something that you know to be wrong, that they know is wrong, but they are acting like it's not wrong - this generates cognitive dissonance), or annoyances like people disagreeing with you on something you're passionate about.

Some annoyances cause this reaction because they present potential physical danger (or something that your brain thinks might be danger). Examples might be a loud irritating noise, or an internal problem like hunger, lack of sleep, pain, etc. 

These and many other triggers make up the concept of "annoyance," and the stress response causes you to feel less relaxed and more vigilant (that feeling of being on edge), and somewhat blocks the transmission of certain relaxing neurotransmitters, like seratonin and dopamine. 

As to pet peeves and other individual differences in what people find annoying, this depends a lot on the learned pathways on your brain. If something has presented more serious issues for you in the past, or something frequently presents problems for you, it's more likely to annoy you more strongly because the pathways in your brain relating to that thing have gotten more "exercise," so to speak. Also, some things may cause stronger cognitive dissonance in some people than others, based on their unique set of thoughts and beliefs, causing greater annoyance when they are violated. Lastly, some people's stress responses are more finely tuned to be triggered by certain things, while others are more numb to the same thing. You can imagine how, if you grow up in a really noisy household, your stress response will probably not be as sensitive to noise as someone who grew up in a really quiet home, because you will be more used to it, and your brain will have learned that it is not really a danger or a hindrance.