“Song” by Lady Mary Wroth presents the feelings of love as things that need to be controlled, whereas in “If thou must love me,” Elizabeth Barrett Browning presents feelings of love as partaking of what is eternal.
There's no doubt that both of these two poets give us an interesting take on the well-worn subject of love. In the case of “Song” by Lady Mary Wroth, we're presented with a vision of love that is cool and restrained. Indeed, at times, it seems almost bloodless in its lack of emotion.
This is because the poem is intended by Pamphilia as a warning to Amphilanthus about the dangers of love. Love is personified as a crying baby that's never satisfied, no matter how much attention it is given. The speaker's cynicism about love grows with each new stanza. Love will be the cause of Amphilanthus's failing; he will triumph in his wailing. On the whole, he will make Amphilanthus suffer.
The speaker's cynicism culminates in the final stanza, where she recommends that Amphilanthus should leave this “child” crying. It's hard to imagine a less conventional treatment of love in a poem, even one dealing with the pain and emotional suffering that love often brings.
In “If thou must love me,” we are given a more conventional presentation of love. Here, love is given to us as eternal, pure, something that will always last. The speaker doesn't just want any old love; she doesn't want a love that's based on her looks, which as she knows full well, will eventually fade.
Instead, she wants a love that will last forever, a love that will not, and cannot, be unwrought by time. This means that she doesn't want her beloved to proclaim his love for her on the basis of her smile or how she looks, things that are subject to change over time. As being ephemeral—here today and gone tomorrow—they are also cliches, and the speaker has no time for them.
In the final stanza, the speaker states emphatically that she doesn't want to be loved out of pity. Nor is she to be loved for herself, but purely and solely for the sake of love:
But love me for love's sake, that evermore
Thou mayst love on, through love's eternity.
Only in this way will their love be eternal.