What is he saying?
"That thou hast her it is not all my grief, / And yet it may be said I loved her dearly;"
The fact that you now have my mistress is not the only thing that causes me pain, although I did love her very much;
"That she hath thee is of my wailing chief, / A loss in love that touches me more nearly."
I'm more upset that she has stolen you from me.
"Loving offenders thus I will excuse ye: / Thou dost love her, because thou know'st I love her;"
Here's the reason why I forgive you both: you love her because you know I love her
"And for my sake even so doth she abuse me, / Suffering my friend for my sake to approve her."
And she is with you because she knows I love you, and she can love me better with your blessing.
"If I lose thee, my loss is my love's gain, / And losing her, my friend hath found that loss;"
If I lose one of you, it means that the other of you gains.
"Both find each other, and I lose both twain, / And both for my sake lay on me this cross:"
You're together and I've lost my relationships with both of you, but you are doing it for my own good.
"But here's the joy; my friend and I are one; / Sweet flattery! then she loves but me alone."
But since the fair lord and the poet are so close as to be the same person, the mistress loves only one person: the poet.
Why is he saying it?
Sonnet 42 is the final of a set of three sonnets that address the fair lord's transgression against the poet: stealing his mistress. This offense was referred to in Sonnets 33-35, most obviously in Sonnet 35, in which the fair lord was called a "sweet thief." This same imagery is used in Sonnet 40, when the speaker says, "I do forgive thy robbery, gentle thief." Sonnet 41 implies that it is easy for the speaker to forgive the fair lord his betrayal, since it is the mistress that "woos," tempted by the fair lord's beauty just as the speaker admires it.
The word "loss" is pervasive throughout this poem, appearing six times. The repetition of this word emphasizes how strongly the speaker feels that he has been deprived of the two most important relationships in his life: the fair lord and the mistress. In the first 4 lines honestly lay out his complaint, while the rest of the sonnet tries to use reasoning to make the betrayal a joyful event: if it is out of love for him that they both betrayed him, he has not lost either of them at all.
While the word "loss" dominates the poem, it is balanced by the word "love," which also appears six times. They even appear together in line 4, "A loss in love that touches me more nearly," referring to the poet's loss of the fair lord to his former mistress. Again in line 9, the two words are woven into the same line, "If I lose thee, my loss is my love's gain," demonstrating how tied together the two ideas are in the speaker's mind.
The use of the term "loving offenders" in line 5 can have two meanings: that the offenders (the fair lord and the mistress) are in love; but it can also mean that they seem to enjoy their offense. This line, along with line 12, "And both for my sake lay on me this cross," hearken back to Sonnet 34, in which the speaker declares, "The offender's sorrow lends but weak relief / To him that bears the strong offence's cross." The biblical allusion of the cross ties the poet himself to Jesus in his suffering so that others might be happy, relieved of their sins.
Though the final couplet seems to turn toward happiness, that happiness is feigned. The idea that the fair lord and the poet are "one" is common in the sonnets; for example, it was asserted in Sonnets 36, 39, and 40. This weak reasoning, that since the poet and the fair lord are one, by loving the fair lord the mistress in fact loves the poet, is not sincere. The exclamation "Sweet flattery!" indicates this sarcasm, since "flattery" always indicates dishonesty or a false beauty, which does not compare to the true love the poet speaks of.