In speaking of both life and death, images from the natural world (bear, iceberg, field daisy) are central to the way the speaker communicates in "When Death Comes." For her, the natural world is not only an avenue through which we can (and should!) observe these forces at work, but it's also our home, since we are part of "a brotherhood and a sisterhood" of living things. We get the feeling that, when the speaker compares each life to a flower, part of what she's doing is putting all forms of life on equal footing (whether flower or person or fish), telling us, in another way, something that she declares a few lines later – that "each body" is "something precious to the earth" (lines 19-20).
By comparing each life to a flower, and declaring each body precious, the speaker makes an argument for the equal worth of all living things.
As seen particularly in the image of the bride and bridegroom, and the last line of the poem, the speaker longs for a stronger connection to the world, which she hopes to accomplish by examining the world until the name of each thing is "a comfortable music in the mouth."