Just reading the title I expect the poem If thou must love me, let it be for nought  to deal with love and the right form of such love. In addition the reader understands the speaking voce is a first person expressed by the object pronoun “me” and  the second character is the person who loves

The poem is an English sonnet organized into three quatrains and a couplet.   

The poem tells about the speaking voce’s opinion about love that it is differentfrom her lover. He loves her for “her smile- her look”, for a  similar  way of  thinking and the sense of peace he feels when he stays with her.  On the contrary, the speaking voce underlines the change of this tattitude in time and how  love could end. For the poet the only way towards eternal love is love for love.

The function of the quatrains is to underline the different type of love and the poet’s position.

In opposition to the quatrains the couplet is the conclusion, the speaking voce expresses what type of love she wants to receive.