They show no hesitation in giving their word nor any refusal to swear an oath, yet not a single promise made by them appears true to me.

This couplet captures the quiet disillusionment of broken trust. The beloved does not hesitate for a second to make big promises or swear sacred oaths, but that very casualness makes their words feel hollow. Having been hurt or deceived before, the speaker finds that the easier a vow is given, the harder it becomes to believe.