Lacking Something Fundamental: Happiness
Previous to her diagnosis, Shell had been active. Now, bedridden, at times even unable to move, and afflicted by paralysis and anxiety, she felt acutely a different sort of pain: depression. Shell mentioned that during this period that she’d been “desperate for serotonin”; serotonin is essentially the means by which the brain regulates mood. A lack of serotonin leads to a host of unpleasant symptoms — the foremost of which is crushing depression.
In the slang dictionary of Gen Z, “serotonin” is commonly used to refer to the general concept of happiness or general positivity. By saying that she was “desperate for serotonin,” then, Shell more or less meant that she was desperate for some sort of positivity.