What was the thought of Shakespeare in "Sonnet 55"?

This excellent sonnet is preoccupied with a common concern
that runs throughout Shakespeare's collection of Sonnets: how
poetry can give immortality to the beauty and love of the person that the sonnet is
written to. Note how this sonnet begins by comparing the ability of this poem to
preserve the beauty of the loved one to the "gilded monuments / Of princes." The speaker
asserts that:

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But you shall shine more bright in these
contents 
Than unswept stone besmear'd with sluttish
time.

The poem here is being
compared to one of those "gilded monuments" in its ability to preserve. However, the
poem will exist eternally, compared to the monuments that, through the ravages of
"sluttish time" are destined to destruction. As the poem continues the speaker develops
this theme. War will overturn statues and destroy them, rendering the memory of the
great people they were supposed to commemorate obsolete. However, being captured in
verse spares the memory of the loved one. The final couplet clinches this theme of the
poem:

So, till
the judgment that yourself arise, 
You live in this, and dwell in lovers'
eyes.

Being remembered
through this poem assures that the loved one that is addressed in this poem will "live
on" until the end of time itself and judgement day.