To suggest the Emily Dickinson thought a lot about death would be an understatement of considerable proportions. Her poems, almost all of which were only discovered after her death in 1886, are all that we have to fill in the substantial gaps in her biography. Not much is known about Dickinson's life, except that it was spent in Amherst, Massachusetts, and that she was regarded as reclusive. That the body of literature discovered posthumously by her sister is so replete with references to death, then, is to present a portrait of a very depressed individual, although how accurate an assessment that is may never be determined.
When reading Dickinson's poems, it becomes apparent that death in and of itself may not have been the preoccupation so much as what comes after death. In her poem "I Died for Beauty" (title provided posthumously from the opening line), Dickinson wrote:
"I died for beauty but was scarce/ Adjusted in the tomb,/ When one who died for truth was lain/ In an adjoining room...And so, as kinsmen met a night,/ We talked between the rooms,/ Until themoss had reached our lips,/ An covered up our names."
Compare that with the following from "Because I could not stop for Death":
"Because I could not stop for Death -/ He kindly stopped for me -/ The Carriage held but just Ourselves -/ And Immortality...Since then - 'tis Centuries -- and yet/ Feels shorter than the Day/ I first surmised the Horses' Heads/ Were toward Eternity."
Dickinson's preoccupation with death and the possibility of an afterlife seemed to haunt her. In a website the link to which follows, a letter Dickinson wrote to Abiah Root would seem to clarify to what she was referring in her poem:
"Does not Eternity appear dreadful to you...I often get thinking of it and it seems so dark to me that I almost wish there was no Eternity. To think that we must forever live and never cease to be. It seems Death which all so dread because it launches us upon an unknown world would be a relief to so endless a state of existence."
Dickinson would appear to have rested more comfortably had she been convinced of the finality of death. It was fear of the unknown to which she was obsessed, and about which the poem "Because I did not stop for Death" was written.