Because I could not stop for Death –
He kindly stopped for me –
The Carriage held but just Ourselves –
And Immortality
We slowly drove – He knew no haste
The speaker felt she wasn’t ready to die, that her life wasn’t over yet, but  personified  Death — clearly a polite gentleman — had other plans for her and felt that her time had come. 
 Dickinson pointedly intertwines the themes of love and death by portraying a man’s courtship as nothing short of death for the female speaker. As the following stanza explains, the woman waits to be called upon, but once chosen must give up both her work and leisure; in short, her identity. 
 Note that ‘Death’ is capitalized, as are the nouns throughout the poem. This gives them added significance, an abstract meaning beyond the everyday definitions. 
 This opening may also reflect the poet’s time and place. Dickinson’s New England would have been a bastion of the severe old-school  “Protestant work ethic” ; perhaps the poet implies ironically that she as a member of this society can’t even stop for the strongest force, death. (Notice that death, by contrast, knows “no haste.”) 
 Dickenson introduces a pattern of hyphens that create  caesurae , forming breaks in the rhythm that may be an expression of fear or uncertainty.
Because I Could Not Stop for Death