In Sonnet 18, the speaker claims that the object of his affection is more lovely and more temperate than a summer's day, more beautiful and pleasant (not too hot, not too cold). But it is the speaker's lines, the poem itself, that make the young man's beauty immortal:
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.
In Sonnet 55, the speaker has the same idea: that his poetry will immortalize the young man. It will outlast marble and "gilded monuments." Time, destruction from wars, and erosion will eventually fade such monuments. The speaker goes further, saying the young man, even after death, will be like a living memory (in verse) as he will "pace forth" and continue to be praised until the end of time. The young man will live on in the poem until Judgment Day when he too will "arise" (go to heaven).