Through scripture we have many instructions on how to grow friendships but between David and Jonathan he have a very real example.

I Sam 18:1-9; I Sam 19.1-7; I Sam 20

I Sam 31:1-6 the death of Jonathan
II Sam 1: the eulogy of Saul and Jonathan by David
David and Jonathan were (I Sam 18:1) "Knit to the soul". Compare with Deuteronomy 13:6-9 "or thy friend, which [is] as thine own soul"

I Sam 15:28 A dire message to Saul and through Ch 16- 19 Saul time after time we are told that Saul despised- hated David.

I Sam 18 Jonathan loved David as he loved his own soul (like Eph 5:25, 28)

I Sam 18:4 Jonathan is acknowledging that David will be the next king

One thing to remember is that David is probably 20 or so Jonathan was very likely 50 or so

A covenant is nothing you do for me it is something I do for you

The limitation of human friendships. Human friendships will end and are not perfect. Look to the friend who will last forever, Matt 11:19, the friend of publicans and sinners