Shakespeare's Sonnet 49

Against that time, if ever that time come,When I shall see thee frown on my defects,When as thy love hath cast his utmost sum,Call’d to that audit by advis’d respects;Against that time when thou shalt strangely pass,And scarcely greet me with that sun, thine eye,When love, converted from the thing it was,Shall reasons find of settled gravity;Against that time do I ensconce me here,Within the knowledge of mine own desert,And this my hand, against my self uprear,To guard the lawful reasons on thy part:To leave poor me thou hast the strength of laws,Since why to love I can allege no cause.

I prepare for that time, if it ever comes, when I see you frown at my faults. Your love will have counted up its worth, called to account by careful thought. I prepare for the time when you pass me like a stranger. You will barely greet me with the sunshine of your eyes. Your love, changed from what it was, will find serious, well-considered reasons to leave.
To face that time, I take shelter in knowing how little I deserve. I raise my own hand against myself to defend the fair reasons on your side.
You have the full force of law behind your choice to leave poor me. I can offer no reason why you should love me.