The Shakespeare's Sonnet's page comments on Sonnet 31:
"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 the Christian might also read this as about finding all those one loved who have indeed died in God, the Beloved:
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 remov'd 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 lov'd, I view in thee,
And thou (all they) hast all the all of me.