The hypothalamus, the master controller of your hormones, releases something called corticotropin-releasing hormone.
This triggers the pituitary gland, a pea-sized gland found at the base of the brain, to release adrenocorticotropic hormone which then stimulates the adrenal gland sitting on top of the kidneys to release cortisol, the major stress hormone.
These natural chemicals are a great help when you need to run away quickly, or do superhuman feats of courage, but when you're simply sitting, these stress hormones collect in the body and affect your overall health.
Stress hormones increase inflammation in the body, suppress the immune system, which makes you more susceptible to infection by acne-causing bacteria, and can even increase oil production in the skin.
And this is the perfect storm for forming a pimple.
Cortisol is a major stress hormone involved in making skin cells churn out oily lipids from special glands called sebaceous glands.
But when there's too much of these oily lipids, called sebum, they can plug up the swollen, inflamed pores and trap the pesky, acne-causing bacteria inside, where they set up house and thrive.
Add a dash of inflammatory neuropeptides released by the nervous system when you're, well, nervous, and angry zits follow.
To make matters worse, Justin is a boy, meaning he's got more testosterone than girls.
Testosterone is another hormone that increases oil production in the skin.
So, his already oily skin, together with a boost in oil and inflammation from stress, is the perfect environment for bacteria to swell, swell, swell up into a major zit.
So what could've Justin done to avoid the big pimple?
Stressful situations are unavoidable.
But we can try to change our responses so that we're not so stressed in the end.
And had he been confident in approaching her, she might not have noticed the pimple, or he might not have had one.