In Shakespeare's Sonnet 139, the speaker begins by expressing his unwillingness to justify his lover's wounding him. He requests that she not look at others in his presence, as it pierces through his heart to see. Instead, he wants her to tell him the truth, to "wound me not with thine eye but with thy tongue." He asks her to tell him the truth, that she loves others, but act as if she loves him. When she's around him, he requests, she should only have eyes for him. He attempts to convey just how pained he is by her behavior.
The speaker then gives his love a way out by making excuses for her obvious behavior: "Let me excuse thee." It makes him feel better to lie to himself and say that she only looks at others because she knows it will hurt them instead of him. He justifies that since she knows her glances have the power to inflict injury, and she does not wish to harm him, her choice is to turn those looks on his foes. In that way, they will receive the harm and the speaker will be shielded.
His relief is short-lived, as the speaker has second thoughts and asks his love to not glance at others after all. He begs her to turn her eyes back to him, even though they are painful. He states that since he is "near slain," she should just turn her attention to him and "rid my pain" since he will have to face the truth in her eyes if she looks at him. These last lines reflect the speaker's admission that he can no longer make excuses. Her blatant disregard for his feelings is destroying him, so he pleads with her to just end his pain with truth. He can no longer watch from afar as she is disloyal to him.