וַיְהִ֗י כְּכַלֹּתוֹ֙ לְדַבֵּ֣ר אֶל־שָׁא֔וּל וְנֶ֙פֶשׁ֙ יְה֣וֹנָתָ֔ן נִקְשְׁרָ֖ה בְּנֶ֣פֶשׁ דָּוִ֑ד ויאהבו [וַיֶּאֱהָבֵ֥הוּ] יְהוֹנָתָ֖ן כְּנַפְשֽׁוֹ׃
And it came to pass, when he had made an end of speaking unto Saul, that the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul.
Read Jewish philosophical commentary on I Samuel 18:1: and classical philosophical sources.