True love is immutable and eternal; it cannot alter with circumstance or be subject to time, and if this be error, I have never written truthfully or loved rightly.
The opening quatrain defines love through negation: 'love is not love' if it 'alters' or 'bends.' The speaker stakes absolute claims, then doubles down with the 'ever-fixed mark' that 'looks on tempests and is never shaken.' The final couplet makes this a wager: if his definition of love is wrong, then he has 'never writ' anything truthful. The sonnet's entire credibility depends on this claim being true.
The sestet confronts Time directly, denying that Time is love's fool. Though 'rosy lips and cheeks' wither within 'his bending sickle's compass,' love 'alters not with his brief hours and weeks' but 'bears it out even to the edge of doom.' This is love as metaphysical principle transcending bodily decay. The image of the sickle (death's weapon) gives urgency to the claim.
You've been with someone through job loss, family crisis, moving countries, and you realize that what you have isn't excitement that fades—it's a bedrock that gets stronger under pressure. It's not about luck or circumstance; it's structural.