Death is personified in this poem. He is presented as a kind of chauffeur and then a seducer. But the journey is slow and leisurely, so one implication is that death is natural, even peaceful. And the personified Death is likewise a guide as much as he is a thief of life; death itself.
That being said, the opening lines suggest that one is never prepared for death.
Because I could not stop for death -
He kindly stopped for me -
The entire poem is the speaker's narration of her journey with death (or Death). This journey has lasted centuries. This is noted in the final stanza. This could mean that death is timeless or endless. The speaker is basically wandering while watching familiar signs of life. Since this centuries long journey only feels like a day, it is as if time stopped and she is now an immortal observer.
The word "passed" occurs four times. Note the pun on "past." The speaker says that the sun passed them, meaning the sun sets more quickly than they move. The speaker, Death and Immortality are outside of time. Everything to them is "past" or, everything passes them by. In this context, a potential thesis statement would be something dealing with time in life and in death. Might Dickinson be presenting an odd afterlife? Rather than heaven or hell, the dead are "past" (beyond) time, like ghosts are beyond the physical world, aimlessly (and yet peacefully) wandering.
“Through Dickinson's precise style of writing, effective use of literary elements, and vivid imagery, she creates a poem that can be understood universally as the acceptance of death.”