Sonnet XXXI

Thy bosom is endeared with all hearts,
Which I by lacking have supposed dead;
And there reigns Love, and all Love's loving parts,
And all those friends which I thought buried.
How many a holy and obsequious tear
Hath dear religious love stol'n from mine eye,
As interest of the dead, which now appear
But things removed that hidden in thee lie!
Thou art the grave where buried love doth live,
Hung with the trophies of my lovers gone,
Who all their parts of me to thee did give,
That due of many now is thine alone:
   Their images I loved, I view in thee,
   And thou (all they) hast all the all of me.

This expands the thought of the previous sonnet. On thinking of his friend, all sorrow for former loves vanishes. Now he supposes that this is because the parts of all the former lovers, and his share in them, is diverted and migrated into the heart of his beloved. Therefore there is no loss at all, for all is stored up within that one gentle heart, and his former love for others was but a prelude to the love he now feels for the youth, and the accumulated devotion he felt for them is now transferred to his beloved's heart. (But there may also be sexual double entendres included to leaven the seriousness). 

This sonnet is important because of its links to A Lover's Complaint, especially the stanza

 'Lo! all these trophies of affections hot,
Of pensived and subdued desires the tender,
Nature hath charged me that I hoard them not,
But yield them up where I myself must render,
That is, to you, my origin and ender:
For these, of force, must your oblations be,
Since I their altar, you enpatron me.   LC 218-224.

which implies that previous loves are somehow transferable to the present one.  Links and echoes such as this one indicate that A Lover's Complaint is indeed by Shakespeare, and not by some other substitute author.  The religious imagery is significant and links in with many of the other sonnets.  See especially the Introductory notes

The 1609 Quarto Version

Thy boſome is indeared with all hearts,
Which I by lacking have ſuppoſed dead,
And there raignes Loue and all Loues louing parts,
And all thoſe friends which I thought buried.
How many a holy and obſequious teare
Hath deare religious loue ſtolne from mine eye,
As intereſt of the dead,which now appeare,
But things remou'd that hidden in there lie.
Thou art the graue where buried loue doth liue,
Hung with the tropheis of my louers gon,
Who all their parts of me to thee did giue,
That due of many,now is thine alone.
   Their images I lou'd, I view in thee,
   And thou(all they)haſt all the all of me.

Commentary

1. Thy bosom is endeared with all hearts,
Thy bosom = Your heart. The seat of emotions was variously considered to be the heart, breast, bosom. Mind was usually reserved for intellectual concerns, and the liver was sometimes thought to be the seat of passion.
endeared = made dear, made precious; is dear to, is loved by all hearts.
2. Which I by lacking have supposed dead;
by lacking = since I no longer have them. The idea is that these former friends and loves, are effectively, or in reality, dead, since he no longer sees them or has contact with them.
3. And there reigns Love, and all Love's loving parts,
Love - Cupid, the god of love (or Eros). Q gives a capital L for the two loves of this line, though it is not retained by all editors. The meaning of 'love in the abstract', and 'my love for you' is also probably intended. all Love's loving parts = all the attributes of love, all the things love shares in. If this line were included in the procreation sonnets, I think we would be compelled to assume a sexual meaning in addition to the more prosaic one. I cannot claim that I really know what is intended by this phrase, and the glosses of 'aspects, attributes, qualities etc.' which GBE, KDJ and JK give all seem rather anodyne. The attributes of Love as Cupid are blindness, softness of limbs, charm, fickleness, innocence, desire, passion, sportiveness, as well as the traditional bow, quiver and arrows. Love as Venus has more serious qualities, but often devastating, where passion overspills the boundaries of mere human control. (See Euripides' Hippolytus for the classic example). So it becomes difficult entirely to dispel the suggestion that Love's loving parts refers, obliquely at least, to physical love and genitalia. It is only because of the tone of the remainder of the sonnet, I think, that we bury this suggestion, as it treats of dear religious love, holy and obsequious tears, the grave of buried love, and it would be sacriligeous to bring such matters of bodily parts into the sanctity of the temple. Yet the phrase occurs again in l.11, and we find ourselves asking 'What is it that the poet is trying to say to the young man, or to us, the readers?' I think that there is a slightly cheeky suggestion which deliberately undermines the seriousness of the sonnet, implying that 'You have indeed all my love, all love past and present, and all parts of it besides, so beware, for you might not know what all those parts really are, and you might get more than you bargained for'. Shakespeare does sometimes use the word with the meaning 'private parts', as in the following:
HAM. Then you live about her waist, or in the middle of her favours?
GUILD. Faith, her privates we.
HAM. In the secret parts of Fortune? O most true! She is a strumpet.
Ham II.2.238-9.
None our parts so poor, but was a race of heaven.
AC.I.3.36-7.
As also in some of the comedies.
4. And all those friends which I thought buried.
which = whom; buried - this has the fianl -ed pronounced, as do endeared and supposed.
5. How many a holy and obsequious tear
holy and obsequious tear = a tear shed in prayers and devotions, or at the funeral rites (obsequies).
6. Hath dear religious love stol'n from mine eye,
dear religious love = love which is precious, and scrupulous in its observance of duty.
stolen - the idea of theft may arise from the thought that the tears are involuntary, therefore love has caused them to flow without the owner's consent. Or it may be that the tears are stolen as being obtained on false pretences, since all those friends for whom tears were shed continue to live hidden in thee.
7. As interest of the dead, which now appear
interest of the dead = interest payable to the dead. Sorrow is owed to precious friends who are dead. This sorrow is the capital, on which interest is payable in the form of tears.
8. But things removed that hidden in thee lie!
But = only, merely;
things = all those friends from line 4 above. things in Shakespeare's day did not only refer to inanimate objects.
removed = that have moved away.
9. Thou art the grave where buried love doth live,
grave = grave and monument. The familiarity with graves, headstones, tombs, and monuments which widespread mortality ensured for most of the population would have made this almost a commonplace image, and not as gloomy as we tend to think it.
10. Hung with the trophies of my lovers gone,

Graves of the nobility would be decked with banners of their coats of arms, and other paraphernalia commemorating their exploits. See the illustration below.
trophies might have a sexual connotation, (my conquests), as it frequently does in Horace. There seems also to be a link to A Lover's Complaint l.218,where the youth is talking of past conquests: 

'Lo! all these trophies of affections hot,
Of pensived and subdued desires the tender,
Nature hath charged me that I hoard them not,
But yield them up where I myself must render,
That is, to you, my origin and ender:
For these, of force, must your oblations be,
Since I their altar, you enpatron me.   LC 218-224. 

See the introductory note above.

11. Who all their parts of me to thee did give,
See the note to line 3 above.
12. That due of many now is thine alone:
So that that love which is (and was) due to many, now belongs only to you.
13. Their images I loved, I view in thee,
Their images I loved = The images of them, which I loved.
14. And thou (all they) hast all the all of me.

And thou (all they) = and you, who have become all of my former lovers in one;  hast all the all of me = have every part of me that is inmost and precious. GBE quotes for comparison Robert Chester, Love's Martyr (1601), p.147:  Thou art that All-in-all that I love best. Compare also For thou art all, and all things else are thine.   LC 266.  This sonnet has many close links with the seductive arguments advanced by the youth in A Lover's Complaint.