Given: Sonnets 18, 73, 116. Explain what these 3 sonnets' closing couplets say about the theme of respect and truth?

Essentially, the theme of the three couplets revolves around the idea of finding, keeping, and discovering a sense of eternal and true love.  In Sonnet 18, the speaker has spent the sonnet comparing the love interest to the natural beauty and temperament of a summer's day.  The closing couplet reaffirms this with its assertion that an eternal sense of natural beauty will never leave the love interest of the speaker.  In linking the beauty of the speaker to a natural setting, it is something that can almost be generally accepted as truth by anyone, indicated in the idea that "So long as men can breathe and eyes can see, /So long lives this and gives life to thee."  In Sonnet 73, the closing couplet emphasizes a similar theme of natural conditions as the backdrop to emphasize a speaker's sense of devotion.  In this setting, while the seasons change to one of frigidity and lack of life, this is cast in stark contrast to the devotion the speaker feels towards the love interest.  While death and decay may be a reality in the natural world, the speaker suggests that this will does not weaken the bond towards the love interest, but rather strengthens it.  The closing couplet, again confirms this, with the idea that "thy love" is strengthened with the belief that while death and termination of life may be a part of the natural setting, it does not apply to the love shared between both speaker and love interest.  This concept of striving to establish something that is permanent in a setting of impermanence, a hope of finding an absolute in a world of contingency, Sonnet 116 stresses that while "impediments" to true love and devotion may abound, the love shared between both parties can transcend this into a realm of pure and accepted truth.  The speaker literally goes "all in" on this notion in the couplet in the suggestion that if proven wrong, "I never writ and no man ever loved."  In all three settings, the commitment and devotion of love is both a part of the world and also seeks to go beyond it as a beacon of light to and for all.