The speaker has been told, by fate itself, that they belong with someone, and they don't want it. The red string of fate is an old image of destined love, and here it glows prettily in the sunlight, but the speaker finds nothing dear in it. They suspect the heavens are "biased and cruel" for making the match without asking.
What stands out is how gentle the rejection is. The speaker uses the familiar "it's not you, it's me" line, calls the other person "a fine lad," and says they mean no harm. Still, the message is firm: the speaker's desire isn't there, and they believe their fate belongs to them, not to "matchmakers in the sky." The final image of metal blades clicking shut suggests snipping the string, which makes the goodbye feel decisive.
The mood is polite, calm and a little weary. There is guilt and kindness, but also a quiet determination to choose for oneself.