We’re not sure that "Thanatopsis" is a completely sad poem. In fact, we think it has a lot of comforting, hopeful moments. Still, you can’t completely separate death and sadness, and sadness is a theme that crops up all over in this poem. It touches on the sadness we feel when we think about death and the grief people feel when their loved one’s die.
Bryant uses his poem not to get rid of all sadness about death, but to take the terror out of it.
"Thanatopsis" uses vast images of time and space to destroy our feelings of sadness, to crush them under the huge weight of eternity.