A lover makes a short, smiling vow. Addressing his “darling, mistress of my heart,” he says their bond will last in gray or sunny weather—through hard times and good times alike. The promise feels intimate and a little private, as if he is borrowing the language of a wedding and giving it a personal, playful twist.
The last two lines do exactly that. The familiar phrase “what God has joined together, let no man put asunder” becomes “none but a better man shall part.” Only a better man could separate them—and the speaker clearly cannot imagine such a man exists. The mood is light, tender, and self-assured: a confident lover teasing the one he adores, turning a solemn formula into both a compliment and a joke.