The physical circumstances of the narrative voice are clearly described in the poem’s details:  snowy, quiet, dark, etc., but the mood or mental state of the narrative character’s deserves discussion: Is he (she?) depressed, even suicidal?  Or is he simply in a contemplative mood, musing on his life’s “journey” as he takes his evening physical journey home?  He is in a familiar relationship with his horse, personifying it and noting its change in habit (the stopping is itself unusual for the horse, as it is for the passenger).  The contemplative mood of them both is underlined by the quiet and the introspection implied in the scene.  The key phrase is “before I sleep,” indicating a long, tiring travel (again, both spatial and temporal).  The mood might be compared to the few moments before one returns home after an exhausting, busy day, anticipating the rest that can only come after the last few miles are covered.