Memory, it turns out, is one of the major factors mediating the dialogue between sensation and emotional experience. Our memories of past experience become encoded into triggers that act as switchers on the rail of psychoemotional response, directing the incoming train of present experience in the direction of one emotional destination or another.

Sternberg writes:

Mood is not homogeneous like cream soup. It is more like Swiss cheese, filled with holes. The triggers are highly specific, tripped by sudden trails of memory: a faint fragrance, a few bars of a tune, a vague silhouette that tapped into a sad memory buried deep, but not completely erased. These sensory inputs from the moment float through layers of time in the parts of the brain that control memory, and they pull out with them not only reminders of sense but also trails of the emotions that were first connected to the memory. These memories become connected to emotions, which are processed in other parts of the brain: the amygdala for fear, the nucleus accumbens for pleasure — those same parts that the anatomists had named for their shapes. And these emotional brain centers are linked by nerve pathways to the sensory parts of the brain and to the frontal lobe and hippocampus — the coordinating centers of thought and memory. The same sensory input can trigger a negative emotion or a positive one, depending on the memories associated with it. 

This is where stress comes in — much like memory mediates how we interpret and respond to various experiences, a complex set of biological and psychological factors determine how we respond to stress. Some types of stress can be stimulating and invigorating, mobilizing us into action and creative potency; others can be draining and incapacitating, leaving us frustrated and hopeless. This dichotomy of good vs. bad stress, Sternberg notes, is determined by the biology undergirding our feelings — by the dose and duration of the stress hormones secreted by the body in response to the stressful stimulus. She explains the neurobiological machinery behind this response: As soon as the stressful event occurs, it triggers the release of the cascade of hypothalamic, pituitary, and adrenal hormones — the brain’s stress response.

It also triggers the adrenal glands to release epinephrine, or adrenaline, and the sympathetic nerves to squirt out the adrenaline-like chemical norepinephrine all over the body: nerves that wire the heart, and gut, and skin. So, the heart is driven to beat faster, the fine hairs of your skin stand up, you sweat, you may feel nausea or the urge to defecate. But your attention is focused, your vision becomes crystal clear, a surge of power helps you run — these same chemicals released from nerves make blood flow to your muscles, preparing you to sprint. All this occurs quickly. If you were to measure the stress hormones in your blood or saliva, they would already be increased within three minutes of the event. In experimental psychology tests, playing a fast-paced video game will make salivary cortisol increase and norepinephrine spill over into venous blood almost as soon as the virtual battle begins. But if you prolong the stress, by being unable to control it or by making it too potent or long-lived, and these hormones and chemicals still continue to pump out from nerves and glands, then the same molecules that mobilized you for the short haul now debilitate you.

Popova