There are several phrases or lines in this poem that indicate the opposite of death. For one, the speaker refers to his loved one's "eternal summer" (line 9). He is describing all the ways in which his lover is actually better than a summer day, and he says that she is superior to a real summer day because the summer sun can sometimes be too hot, or the winds can be too rough, and the season of summer only lasts a short while. His lover, on the other hand, will enjoy an "eternal summer" as a result of the lines he writes about her.
Further, he says that death will not "brag thou wander'st in his shade" because she will continue to live on in his verse. Here, he personifies death as someone who might boast about having captured such a beautiful woman and then keep her in relative darkness with him (as opposed to the sun, which is associated with life in this poem). The speaker says that his "eternal lines" will last forever, allowing his lover and her beauty to live forever in them. He believes that he immortalizes her with this poem. In the end, he claims,
So long as men can breathe or eyes can see, / So long lives this, and this gives life to thee. (13-14)
In other words, as long as people are still alive in the world, this poem will continue to live on as well and the poem gives his lover eternal life.