Sometimes looking at the structure of the poem helps to add understanding to its meaning. Sonnets are 14 lines of iambic pentameter which brings structure and meter, but there also tend to be patterns withing that structure that also help. For example, the first quatrains generally pose a problem and the ending couplets resolve it. In these two sonnets it seems like he makes his points throughout the poem when he says that his lady and love are NOT what everyone thinks they are. In Sonnet 18 he discusses in the quartrains that his lady isn't beautiful in many ways that the world would consider lovely and he can't stop the march of time. In Sonnet 116, he discusses what love is not in the quatrains which challenges human faultiness when dealing with love. Then, he drives home his final "say" on the matter in the couplets. So, in 18, he finds a way to defy time by solidifying his lady's memory in a poem and in 116 he challenges anyone to disagree with his position about love by saying, "If this be error and upon me proved,/ I never writ, nor no man ever loved." Summarily, Shakespeare usually challenges something that he can't change in his quatrains (or approaches a problem) and then makes up his mind about it in the ending couplet.