The friendship of David and Jonathan was the effect of Divine grace, which produces in true believers one heart and one soul, and causes them to love each other. This union of souls is from partaking in the Spirit of Christ.
Where God unites hearts, carnal matters are too weak to separate them. Those who love Christ as their own souls, will be willing to join themselves to him in an everlasting covenant. It was certainly a great proof of the power of God's grace in David, that he was able to bear all this respect and honour, without being lifted up above measure.