1 Samuel 18

We read in 1 Samuel -

"...that the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul." (Old Testament | 1 Samuel 18:1)

Even to the point that -

"...Jonathan and David made a covenant..." (Old Testament | 1 Samuel 18:3)

I'm sure this was a promise of great friendship. In modern day, we read somewhat of the same covenant that the priesthood should have one with another -

"Let him offer himself in prayer upon his knees before God, in token or remembrance of the everlasting covenant.
132 And when any shall come in after him, let the teacher arise, and, with uplifted hands to heaven, yea, even directly, salute his brother or brethren with these words:
133 Art thou a brother or brethren? I salute you in the name of the Lord Jesus Christ, in token or remembrance of the everlasting covenant, in which covenant I receive you to fellowship, in a determination that is fixed, immovable, and unchangeable, to be your friend and brother through the grace of God in the bonds of love, to walk in all the commandments of God blameless, in thanksgiving, forever and ever. Amen." (Doctrine and Covenants | Section 88:131 - 133)

This covenant is no different from the Savior's charge, that is -

"A new commandment I give unto you, That ye love one another; as I have loved you, that ye also love one another.
35 By this shall all men know that ye are my disciples, if ye have love one to another." (New Testament | John 13:34 - 35)