1 Against that time (if ever that time come)
In preparation for that day (if it ever arrives), I'm getting ready for the day when you stop loving me, preparing for when you leave 'Against that time' = in preparation for; the repetition signals obsessive anticipation.
2 When I shall see thee frown on my defects,
When I see you frown upon my flaws, when you start seeing all the ways I'm not good enough, when you despise my failures 'Frown' = show disapproval; 'defects' = moral or personal failings.
3 When as thy love hath cast his utmost sum,
When your love has tallied up the whole account, when you've done the math and figured out I'm not worth it, when you've counted the cost 'Cast his utmost sum' = calculated the total; love becomes accounting, adding up assets and liabilities.
4 Called to that audit by advised respects,
Called to this reckoning by careful reflection, prompted by your clearer, sober thinking, when you finally think clearly 'Audit' continues the accounting metaphor; 'advised respects' = mature, considered judgment.
5 Against that time when thou shalt strangely pass,
In preparation for when you will pass me as a stranger, for the moment you walk past me like I'm nobody, when you treat me like a stranger 'Strangely pass' = pass by as if a stranger; the beloved will become foreign.
6 And scarcely greet me with that sun thine eye,
And barely acknowledge me with that sun that is your eye, and won't even give me the warmth of your glance, won't shine your light on me 'That sun thine eye' = the beloved's bright, warm look; the withholding of this is the ultimate coldness.
7 When love converted from the thing it was
When love, transformed from what it once was, when what you felt for me turns into something else, when your love changes 'Converted' = transformed, changed; love will metamorphose into indifference or dislike.
8 Shall reasons find of settled gravity;
Shall find serious justifications, will find very good reasons to leave, you'll have reasons to go 'Reasons' = justifications; 'settled gravity' = mature, weighty deliberation. The beloved's mind will be made up.
Volta The volta shifts from describing anticipated rejection to the speaker's preemptive self-defense: he fortifies himself with knowledge of his own unworthiness.
9 Against that time do I ensconce me here
Against that day I fortify myself here I'm preparing my defense, taking shelter in i'm bracing myself 'Ensconce' = fortify, entrench; military metaphor suggests the speaker is preparing for siege.
10 Within the knowledge of mine own desert,
Within the fortress of my own self-knowledge—how unworthy I am, in what I know is true about myself, knowing my own worthlessness 'Desert' = worthiness/desert (what one deserves); the speaker claims self-knowledge as a fortress.
11 And this my hand, against my self uprear,
And I raise this hand against myself, and I testify against myself— i'm my own witness against me 'Uprear' = raise up; the hand becomes a weapon pointed at the self, a legal gesture of testimony.
12 To guard the lawful reasons on thy part,
To defend the rightful arguments on your side, to protect your right to leave, i'm defending your case against me 'Guard' = defend; the speaker is not defending himself but the beloved's right to leave.
13 To leave poor me, thou hast the strength of laws,
To leave me, you have the law itself on your side, you have every right, every law backs you up, you have the law on your side 'Strength of laws' = legal backing; the speaker grants the beloved absolute moral and legal authority to depart.
14 Since why to love, I can allege no cause.
Since I can offer no reason why you should love me. because I have no answer to why you should love me at all. i can't give you one reason to stay The couplet's final admission: love is groundless; there is no 'because' for it. Absence of cause = absence of obligation.