The reasons behind Dickinson's introverted/reclusive personality have been debated umpteen times, all the way from agoraphobia (an anxiety disorder) to obsessive compulsive disorder (OCD). Dickinson's fascination with death and suicide - not to mention her gradual withdrawal from the world outside her bedroom door - however seem to suggest that Dickinson was suffering from extreme depression. Further research confirmed that depression/bipolar disorder could indeed have been behind Dickinson's eccentricies.
It would be hard to pin down though whether it is the writer's solitary nature that enables depression to gather a stronghold on their personality, or whether inherent depression in fact causes a writer's need to be solitary from the start. How frustrating is that? As a writer with potentially depressive tendencies myself, I'm crossing my fingers in the hope that the link between the imagination and depression will be explored in greater depth in the future, if only to stave off the spectre of macabre melancholy for some moments longer:)
This leads me to the most troubling thing about depression - other than turning reclusive (which I do tend towards hmmmm). Sadly, suicide is a scarily real alternative for those suffering from depression. Writers are certainly not exempt from this dangerous aspect to the dark well of nothingness that depression can cause, as a certain letter from Kiana Davenport to JA Konrath shows. You might want to have a look too at a few other articles/posts on this correlation between writers in particular and depression: Writers and Depression, Writers 'at greater risk of depression' survey finds, and a personal piece by Nancy Etchemendy.