וַיִּכְרֹ֧ת יְהוֹנָתָ֛ן וְדָוִ֖ד בְּרִ֑ית בְּאַהֲבָת֥וֹ אֹת֖וֹ כְּנַפְשֽׁוֹ׃
Then Jonathan made a covenant with David, because he loved him as his own soul.
Read Jewish philosophy on I Samuel 18:3: Rambam's Guide for the Perplexed, Saadia Gaon, and classical Jewish philosophical sources, free online.