“The Bargain” is part of Sir Philip Sidney's pastoral romance Arcadia, and it presents a love relationship through the imagery of an exchange of hearts. Let's look at this more closely.
The speaker in the poem is a shepherdess, and she is speaking of her beloved, a shepherd. She begins with the claim, “My true love hath my heart, and I have his.” We know this cannot be literally true, of course, so the shepherdess is speaking metaphorically. She and her beloved have made a bargain. She has given him her heart in exchange for his, and she believes that “never was a better bargain driven.”
Notice the economic language and imagery here. We don't normally speak of love as such a bargain, so the audience is both a little startled and quite intrigued. The speaker goes on to explain that she holds her beloved's heart “dear,” and he “cannot miss” hers. Indeed, because they have exchanged hearts, they live in perfect unity. They are one. The speaker's heart guides her beloved's “thoughts and senses.” He thinks of her when he speaks and acts. And the speaker “cherishes” the heart of her beloved.
Indeed, this relationship is one of full unity, so much so that each lover is fully conscious of the other at all times. They will never break their bargain, for if they do, perhaps both their hearts will break along with it.