The words "emotion" and "passion" do not appear in William Cullen Bryant's poem "Thanatopsis," so I have to assume you are referring to the overall tone and feeling of the poem. This is a poem about death and dying, but it is not particularly depressing or morbid. In fact, one is almost encouraged about the subject after reading the poem. The primary emotion of the poem is comfort in the face of inevitable death.
The opening lines of the poem are a praise of Nature when life is good and death is far away; the speaker then reminds us that it is the same Nature who will embrace us when it is our time to die. Despite the change in tone from "gayer hours," "gladness," and "eloquence of beauty" to "bitter hour," "blight," sad images," "stern agony," and the "breathless darkness" of a "narrow house," Nature's voice is still calm, comforting, and omnipresent.
Death, the speaker says, is part of every man's existence. Once a man has gone to the "narrow house" of his grave, he will become one with all of the natural elements which surround him.
Earth, that nourished thee, shall claim
Thy growth, to be resolved to earth again,
And, lost each human trace, surrendering up
Thine individual being, shalt thou go
To mix for ever with the elements.
He will become brothers with stones and dirt and tree roots, just as everyone who died before him has done and as everyone who dies after him will do. The speaker reminds us the final resting place below ground is a magnificent place to spend eternity because of all the other people we will join there.
Thou shalt lie down
With patriarchs of the infant world—with kings,
The powerful of the earth—the wise, the good,
Fair forms, and hoary seers of ages past,
All in one mighty sepulchre.
This "great tomb of man" is, in reality, much greater and grander than anything here on earth, for there are many more people there than those who are currently alive on the planet. There is great comfort in that.
One of the fears people have about death is that no one will mourn them when they are gone. The speaker of the poem addresses that, as well, by reminding us that it does not matter whether anyone weeps or even notices that we are gone. Let the living continue with their lives, and if they are happy, so much the better. No matter what their lives are like, they will all one day join the dead.
All that breathe
Will share thy destiny. The gay will laugh
When thou art gone, the solemn brood of care
Plod on, and each one as before will chase
His favorite phantom; yet all these shall leave
Their mirth and their employments, and shall come
And make their bed with thee.
While the majority of the poem is a comforting reminder about the inevitability and universality of death, the final lines of the poem are an encouragement about living well until that time comes.
So live, that when thy summons comes to join
The innumerable caravan, which moves
To that mysterious realm, where each shall take
His chamber in the silent halls of death,
Thou go not, like the quarry-slave at night,
Scourged to his dungeon, but, sustained and soothed
By an unfaltering trust, approach thy grave,
Like one who wraps the drapery of his couch
About him, and lies down to pleasant dreams.
Death is not something to be feared or scorned, as it is part of the shared human experience. We are not to live like prisoners, enslaved by our fears of death; instead we are to see death as a welcome end to a life well lived.