Sonnet XLII

That thou hast her it is not all my grief,
And yet it may be said I loved her dearly;
That she hath thee is of my wailing chief,
A loss in love that touches me more nearly.
Loving offenders thus I will excuse ye:
Thou dost love her, because thou know'st I love her;
And for my sake even so doth she abuse me,
Suffering my friend for my sake to approve her.
If I lose thee, my loss is my love's gain,
And losing her, my friend hath found that loss;
Both find each other, and I lose both twain,
And both for my sake lay on me this cross:
   But here's the joy; my friend and I are one;
   Sweet flattery! then she loves but me alone.

This is the last in the trilogy of sonnets devoted to the youth's betrayal of the poet by stealing his mistress. It was traditional that the sonneteer should find excuses for the beloved's behaviour. Her purity (for it was usually a woman) and innaccesibility were part of a higher ideal and she could not bend herself to earthly love. The poet therefore had to justify her aloofness and her cold chastity.

Here the poet has to justify infidelity, a far more difficult task, and set against the background that the beloved should be irreproachable, the traditions of sonnet writing are wittily parodied.

Nevertheless there lingers a bitter aftertaste, for despite all the sophistry of excuse and justification, the poet is left to mull over his loss, and the joy and sweet flattery which he claims to find in the concluding couplet ring hollow. They sound very much like gratuitous and foolish self deception. The beloved has everything and his lover, the poet, must content himself with the shadow of a love, a love who has betrayed him and found joy and consolation in the arms of another.

The 1609 Quarto Version

THat thou haſt her it is not all my griefe,
And yet it may be ſaid I lou'd her deerely,
That ſhe hath thee is of my wayling cheefe,
A loſſe in loue that touches me more neerely.
Louing offendors thus I will excuſe yee,
Thou dooſt loue her,becauſe thou knowſt I loue her,
And for my ſake euen ſo doth ſhe abuſe me,
Suffring my friend for my ſake to aprooue her,
If I looſe thee,my loſſe is my loues gaine,
And looſing her,my friend hath found that loſſe,
Both finde each other,and I looſe both twaine,
And both for my ſake lay on me this croſſe,
   But here's the ioy,my friend and I are one,
   Sweete flattery,then ſhe loues but me alone.

Commentary

1. That thou hast her it is not all my grief,
'The fact that you now possess her is not the sole cause of my grief'. to have, in the context of a loving relationship, refers both to the oneness of love, as in line 11 and in
My true love hath my heart and I have his, (Sidney),
as well as to the physical act of coition, as in Sonn 129:
Had, having, and in quest, to have extreme.
Similarly in line 3. The tone of the poem usually indicates which, if any, meaning should predominate. Here there is a constant interchange between the two meanings, as the tide of jealousy and forgiveness ebbs and flows in the poet's mind.
2. And yet it may be said I loved her dearly;
The poet does not wish to belittle his loss. Though the main cause of his sorrow is not the loss of his mistress, as explained in the following lines, it is still not an insignificant or petty sorrow, but it is outweighed by other considerations.
dearly = with heartfelt affection; at a cost.
3. That she hath thee is of my wailing chief,
is of my wailing chief = is the main cause of my grief. Perhaps, as SB suggests, a reference to the chief mourner at a funeral.
wailing = sorrow.
4. A loss in love that touches me more nearly.
A loss in love - loss and love are the two key words of this poem. loss and its cognates occur six times, the same number as for love and its cognates. The two are thus evenly balanced, as if one brings the other in its train.
nearly = grievously, deeply, close to my heart.
5. Loving offenders thus I will excuse ye:

Loving offenders = sinful lovers; loving sinners. The phrase encapsulates the psychological difficulty of maintaining a relationship with the two participants. The word offenders is used in Sonn 34 where it relates to episodes from The Passion. 
The offender's sorrow lends but weak relief
To him that bears the strong offence's cross
and this echoes line 12 of this sonnet. How can one forgive the loving offenders who have committed this strong offence? (Although cross in Sonn 34 is an emendation for loss). At the same time in the phrase loving offenders there is probably a suggestion of the proverbial good will which lovers enjoy. 'All the world loves a lover', 'They are offenders, but they are in love', etc.

6. Thou dost love her, because thou know'st I love her;
These two lines (6.7) set out the reason for excusing the betrayal. Briefly it is that both of them did it out of love for him.
7. And for my sake even so doth she abuse me,
even so = in like manner, i.e. because she knows I love you, and therefore by loving you she loves me;
abuse = deceive, maltreat, harm.
8. Suffering my friend for my sake to approve her.
Suffering = allowing, permitting; but with overtones which speak of the poet's own experience of suffering caused by this liaison.
to approve her = to try her out (sexually); to give her his blessing.
9. If I lose thee, my loss is my love's gain,
9-12 expand the thought put forward in 6-8.
10. And losing her, my friend hath found that loss;
And losing her = and in my losing of her.
my friend = my beloved (the young man).
11. Both find each other, and I lose both twain,
both twain = the two together; the two individually. See the use of twain in Sonns. 36 and 39.
12. And both for my sake lay on me this cross:

this cross = this sorrow, this anguish. The imagery is the familiar Christian one of the crucifixion, with the speaker adopting a Christ-like pose of suffering and forbearance. It could also suggest the episode of Simon of Cyrene being forced to carry the cross on the path up to Calvary, a cross which, according to tradition, he at first refused to take up. See the Introductory Note for further discussions of the relevance of the Christian imagery in the Sonnets. 

13. But here's the joy; my friend and I are one;
The final link is placed in the chain of reasoning. Far from being a sorrow, the liaison between the poet's mistress and the beloved youth has become a source of joy.
my friend = my beloved.
one - the unity of lovers is once again stressed. It is a frequent theme in the sonnets. See 36, 39, 40.
14. Sweet flattery! then she loves but me alone.
The dishonest truth of the concluding couplet is underscored by the use of the word flattery, for it always carries overtones of deception, either of another, by false praise, or of oneself, by forcing oneself to believe what one wants to believe, rather than what is true. The instances of its use in the sonnets are listed below. All of them suggest a false stance or a currying of favour, which sits ill alongside the concept of true love which the poet is trying to defend.

So flatter I the swart-complexioned night, 28

Full many a glorious morning have I seen
Flatter the mountain tops with sovereign eye, 33

Thus have I had thee, as a dream doth flatter, 87

..........that my adder's sense
To critic and to flatterer stopped are. 112

Or whether doth my mind, being crowned with you,
Drink up the monarch's plague, this flattery? 114

O! 'tis the first, 'tis flattery in my seeing, 114

And in our faults by lies we flattered be. 138