"Thanatopsis" can be categorized as an elegy, a poem about death that begins with an overall melancholy tone but ends with a more positive, uplifted tone.  There are two major shifts in the poem that are indicated with transition words.  The first comes at line 31 with the transition line "Yet not to thine eternal resting-place / Shalt thou retire alone, nor couldst thou wish / Couch more magnificent" (ll. 31-33).  Bryant started the poem talking about a man who thinks about death and the possibility of dying alone and friendless.  The tone shifts in these lines to assurance that all people will go to the same place when they die, "in one mighty sepulchre," the Earth as tomb (l. 37).  Bryant assures the reader that there we will be buried with people from all walks of life, from the greatest to the least.  The Earth-tomb is the mighty equalizer.

The second shift comes near the end of the poem, again indicated by a transition word: 

So live, that when thy summons comes to join
The innumerable caravan which moves
...
Thou go not, like the quarry-slave at night,
Scourged to his dungeon, but, sustained and soothed
By an unfaltering trust... (ll. 73-74, 77-79)

Bryant gives the reader his advice, to live life looking forward to the great beyond instead of dreading it and being bogged down by thoughts of it.  His view of death is not a religious one but a natural and spiritual one that fits with the Romantic writers of his time.