Just considering the title the reader expects the poem will be about love. It creates curiosity about the condition posed to love somebody. It is not a coincidence that the title is repeated in the first line of the sonnet. This reinforcing the same curiosity. 

Not only does the first line repeat the concept of the title, it puts the message itself into a better focus 

Looking at the lay-out the reader notices that it is a part of a collection because the title coincides with the first line. Besides, the poem is a sonnet that follows the Elizabethan structure consisting of three quatrain and a couplet.

The poem is about a woman who tells her lover to love her just for love because it is the only way to love somebody eternally. On the other hand if somebody loves someone else only for her/his esthetical qualities the feeling will change.

Regarding connotation, in the first quatrain there is an invitation about love; the reader understands it from the imperative forms “let it be” and “don’t say”.

The language used to express the semantic feel is about the way the woman approach the man (smiling, looking, speaking, …). At line 4^th there is an enjambment, so the direct speech continues on the second quatrain.

At line 8^th there is the repetition of the word “change” that underlines the possible change of feelings.

About phonological level the reader notices that when the speaking voice speaks about her idea of love she uses long open vowel sounds (thou, love, for, nought, …) because if a man loves in this way a woman the love last, so the open sound reminds to eternity, while when she speaks about mortal things the rhythm is faster, there are lots of comma and sounds like E, I,…

At line 10^th there is the alliteration in W who reminds to somebody who is crying.

In conclusion I think the message of the sonnet is that if love is just connected to the esthetical qualities it isn’t true love so it doesn’t last. The only way to eternize love is to “love for love’s sake”.