"...when in eternal lines to time thou grow'st"- Explain

This line have been taken from 'Shall I Compare Thee' written by William Shakespeare. The poet here strongly advocated for the perpetual continuity of his friend's beauty.

      The 'eternal summer' of Shakespeare's supposed patron-friend W.H. is referred to here. In these lines the poet powerfully told that unlike the beauty of summer's day, his friend's beauty is eternal and perpetual. It is too endurable to fade. It wiil not vanish by the destructive march of time. The poet here boldly affirms that his friend is the embodiment of eternal beauty. His loveliness frowns at death even. The poet wants to immortalize his friend's beauty through his verse. As long as his verse will last, his friend's beauty will last. Nothing can steal away his summer nor defile the sublimity that his friend is possessed of. The enduring effect of his poetry will ennoble his friend with everlastingness.