system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does you heart "flutter" or skip a beat when you see someone attractive?
gpt
It is an over stimulation of the vagus nerve caused by moments of anxiety or other stress.  In moments of high anxiety your body releases adrenaline and other hormones that excite autonomic functions such as heartbeat.

It is when you encounter that person you are keen on unexpectedly your brain plays out what seems like 100 scenarios simultaneously.  Your body senses stress and "Fight or Flight" mode kicks in by your brain flushing your nerveous system with adrenaline and cortisol, each of which are powerful stimulants.  

The vagus nerve is like an interstate highway for your nerveous system.  It connects most autonomic functions such as heartbeat, breathing, and digestion.  In reality, it's not JUST heart palipitations you experience, but other disruptions such as breathing, digestion, endocrine (sweating) and basically all other body responses.  It's just that in that one instant you are more likely to notice your heart beating out of sync than your digestion slowing or breathing differently.