This is a slightly unusual sonnet for Shakespeare in that, even though the ideas are fairly conventional, the poet essentially threatens his disdainful lover that if she doesn't at least appear to return his love he might have to speak ill of her.
In the first four lines, for example, the poet warns his lover that she should temper her disdain because his sorrow might cause him to say something to her discredit.
Then, he tells her that it would be much better were she to tell him that she loves him, even if she does not, in the same way that men near death are comforted by their physicians, who tell them they are healthy. Clearly, the implication of the threat should be clear to the lover: even if you don't feel love, tell me you love me.
The threat becomes clear in the next four lines when the poet says that "if I despair," and go crazy, I might say something bad about you. Even worse, this world is so full of crazy people that they will believe whatever they hear.
The couplet provides the solution as far as the poet is concerned--keep your pride ("thy proud heart go wide"), but look me straight in the eye and tell me that you love me.