In 'Sonnet 116', Shakespeare presents his ideas about what love is, with reference to romantic love. We can deduce that he writes of romantic love through the phrase "Let me not... admit impediments", which is reminiscent of marriage vows. Shakespeare outlines his definition of love through a series of images, which is developed through his use of the sonnet form.
First of all, Shakespeare believes that love in its truest sense is unchanging. He writes, "love is not love / Which alters when it alteration finds". By stating "love is not love", he attempts to undermine the very definition of 'love', reshaping it into his own ideal. Furthermore, the repetition of words such as 'love', 'alter' and 'remove' show a lack of variety in vocabulary, therefore reflecting the constancy of true love. However, by slightly altering the word in the repetition, such as 'remover' and 'remove', or 'alters' and 'alteration', Shakespeare is highlighting the frequency of changes within relationships, and therefore suggests that few examples of love will survive these and therefore prove themselves to be 'true'.
Moreover, Shakespeare presents love as a nurturing and guiding influence. He writes "It is an ever fixed mark / That looks on tempests and is never shaken", thus likening love to a celestial presence, connoting guidance and goodness. Indeed, by using the metaphor of a "star to every wandering bark", the image of a star adds to the notion of a guiding presence, adding the idea that it gives light, and therefore hope and joy. Comparing a romantic relationship to a "wandering bark" and "tempests" shows Shakespeare's acknowledgement that relationships are not perfect and without their troubles, suggesting that exterior influences can steer them off course. The wholesomeness of love is also developed in the colour imagery of "rosy lips and cheeks", which suggest youth, beauty and health. Here, Shakespeare is suggesting that although a couple may age, if their love remains unchanging, it will remain fresh and youthful. Indeed, by comparing the soft "rosy" colouring to the violent "Time's...bending sickle", he further exaggerates the goodness of his ideal love, and the extremes of external forces, which love must withstand.
Finally, Shakespeare uses the final couplet of his sonnet to state that this belief of 'true love' is ingrained in his identity, and disproving it will undermine his whole being. He writes "If this be error and upon me proved, / I never writ, nor no man ever loved". As writing is Shakespeare's profession and therefore an integral part of his identity, his final statement "I never writ" therefore suggests that his whole understanding of the world and his place within it will be upturned "if this be error". On the other hand, it could be argued that Shakespeare is using a conditional statement next to something which is impossible to undermine or disprove, therefore making it impossible for the reader to disagree with him. Indeed, the rigidity of the structure and iambic pentamenter is reflective of how Shakespeare is unwilling to accept any other viewpoint than his own.
Therefore, in 'Sonnet 116', Shakespeare presents romantic love in a somewhat didactic way, detailing it as unchanging and invulnerable.