and I think of each life as a flower, as common as a field daisy, and as singular,
Our speaker thinks of each life as a flower for two reasons: to recognize its commonness, and also its individuality.
Now it's life (instead of death) that is given an image.
A flower seems a fragile image in the face of death. Flowers are known for being short-lived; they bloom, and they're beautiful, then they're gone.
Still, since the poem has just questioned the nature of death and time, perhaps a flower is no more frail than anything else.
(Plus, the field daisy that exists in this poem and our imaginations lives in a sort of eternity).
The image of the flower also seems to be a way of making all life equal; equally common (there are lots of flowers out there) and equally unique (no two flowers are exactly alike).
From our speaker we get the feeling that a flower's life, in a sense, is no less valuable than a person's.