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He compares her to the being "temperate" and "lovely".

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"and summer's lease hath all too short a date"

this is explaining that as the summer, which he compares his love to, ends too soon.

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"nor shall death brag thou wand'rest in his shade'

This explains how his love will not die, as an eternal summer will not.

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"And every fair from fair sometime declines...but thy eternal summer shall not fade." represents how the person's beauty will never disappear in his eyes.

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"Sometime too hot the eye of heaven shines;/ And often is his gold complexion dimmed"

I think this means that his love is underappreciated and those can't truly see its real value.

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"So long lives this, and this gives life to thee"

This shows how he compares the worth of the summer and his love.