The answer lies in how changes in neurotransmitter levels affect our individual brain chemistry.
As Renshaw's theory goes, serotonin deficiency exacerbates symptoms of pre-existing anxiety and depression, increasing the likelihood of becoming suicidal (mental illness is a factor in about 90% of suicides). People with an existing mood disorder, or a predisposition to mental illness, would be more sensitive to the effects of waning serotonin levels.