Your pupil size is controlled by your autonomic nervous system, which regulates all of your involuntary actions like breathing, heart rate, and sweating. A branch of this system called the sympathetic nervous system, or SNS, is responsible for your body's fight or flight mechanism.
This mechanism acts up when you're under stress, like when you're talking to your crush, and releases the hormone noradrenaline, causing your pupils to dilate. This same hormone also causes you to sweat more and your heart to beat faster around someone you like.
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