Mood, or atmosphere, is the feeling that a literary work generates. Elements that can influence the mood of a work include its setting, tone, and events. The mood of "Because I Could Not Stop for Death" is certainly affected by the tone of the speaker, which is one of confiding. And, the mood created by the imagery in Dickinson's poem does seem to change from the beginning of the poem to its end.
In the first stanza, the speaker seems almost charmed by Death, who with his carriage has "kindly stopped" for her. With a desultory intimacy the speaker and Death are alone in the carriage.
The carriage held but just ourselves
We slowly drove. He knew no haste,....
This peaceful mood dissipates somewhat with images of children who "strove/At recess in the ring," and "fields of gazing grain." Then, the speaker becomes a little tense and senses a loss of control as she contradicts the line "We passed the setting sun":
We passed the setting sun--
Or rather, he passed us.
And suddenly in the third stanza, the mood changes to one of slight tension as the pace of the sentences increase as Dickinson employs alliteration to accelerate the lines:
The dews drew quivering and chill,
For only gossamer my gown,
My tippet only tuille.
After this stanza, the mood grows somber as the speaker describes having stopped before a house that was only "a swelling of the ground." She somberly adds that she was surprised as she "surmised the horses' heads/Were toward eternity."