The reader understands that the poem IF THOU MUST LOVE ME, LET IT BE FOR NOUGHT is a part of a collection because it bears the same title of its first line.
Just considering the title the reader can expect the poem may be about a person talking with his beloved.
The test is a sonnet because it has the Shakespearian sonnet's structure: it consists of three quatrains and a couplet.
In the first quatrain a woman tells her lover to love hers just for love and not only for her body because it is the only way to make their love eternal. In her opinion there isn't love when you love somebody only for "her smile - her look". Here the invitation about love is expressed with imperative forms, such as "let it be" and "don't say".
In addiction in the second quatrain the change of feelings is expressed by the repetition of "change" at line 8.
In the third quatrain there is an alliteration of "-w" to reproduce the crying's sound.
The speaking voice uses open vowel sounds("thou", "love") to speak about love because they remind to eternity.
In my opinion the sonnet's message is that if you love somebody only for esthetical characteristics, isn't true love and it can't last.