1 Samuel 20:15 - Matthew Poole's English Annotations on the Holy Bible

Thy kindness; he saith not, the kindness of the Lord, as before, because the covenant between them seem to have been only personal, and not comprehending their posterity. And therefore as Jonathan speaks confidently of his own preservation by virtue of that covenant; so he here lays this additional obligation upon David, that he would extend his kindness to all his progeny. When the Lord hath cut off the enemies of David; which he will certainly do without exception of Saul my father, and those of his children who have joined, or shall join, with him in his hostilities and cruelties against David. And by this word enemies he implies the reasonableness of his request, because Jonathan was none of that number, and therefore not to be treated as such.