William Cullen Bryant was a Romantic, and like many of the Romantics, he believed that in Nature, one could find spirituality and comfort. In the poem "Thanatopsis," the speaker is first introduced as one who "holds / Communion with [Nature's] visible forms" (ll. 1-2), and thus, she (Nature) speaks to him through Nature, so there is no being alone even when the speaker is without human company.
This poem also speaks about Bryant's view of death and the afterlife; Bryant takes a very natural and almost scientific view of what happens to a body when it dies--it becomes part of "one mighty sepulchre," the Earth as tomb (l. 37). In this "great tomb of man" (l. 45) lie everyone who has come before the speaker: "patriarchs of the infant world,--with kings, / The powerful of the earth,--the wise, the good, / Fair forms, and hoary seers of ages past" (ll. 34-36). So even in death, while the speaker may wish to be alone, he is instead surrounded by humanity and Nature.