1 Samuel 18:1-4
18:1 After David had finished talking with Saul, Jonathan became [note two things that indicate that David and Jonathan enjoyed an intimate friendship]  one in spirit with David [their lives were bound together; what hurt the one hurt the other; “Friendship is one mind in two bodies.” (Mencius)], and he  loved [Heb. verb “aheb”; note: this is not a reference to homosexual desire or activity; the OT employs the word “yada” (“know”) to describe homosexual activity] him as himself.
Note: “The quality of friendship is nearly always determined by the quality of that which we have in common. We share our lives with people who share a like vision. … The deepest interest Jonathan and David shared was the well-being of God’s kingdom (see 1 Sam. 14:6; 17:47). Every link with a friend is useful, but until we are knit together by a commitment to the Lord, we are wading in the shallows of biblical friendship.” (Gary Irving, “Moody Monthly” magazine, 02.79)
18:2 From that day Saul kept David with him and did not let him return to his father’s house.
18:3 And Jonathan made [cut] a covenant [terms of covenant not recorded here; terms apparently included pledge of mutual loyalty and friendship (cf. 20:14-17,42)] with David because he loved him as himself.
18:4 Jonathan took off the robe [a symbol of the Israelite kingdom; cf. 1 Sam. 15:27-28; a symbol of Jonathan’s right to the throne] he was wearing and gave [an expression/indication of Jonathan’s great love for David; a recognition of David’s worth; “Friendship without self-interest is one of the rare and beautiful things of life.” (James F. Byrnes)] it to David, along with [Jonathan gave his all to David] his tunic, and even his sword, his bow and his belt [note that earlier David had refused to receive similar items from Saul (cf. 1 Sam. 17:38-39)].
Note: Dale Carnegie said, “You can make more friends in two months by becoming really interested in other people, than you can in two years by trying to get other people interested in you.”
1 Samuel 18:8-9
18:8 Saul was very angry [unjustified anger; Saul should have been grateful for David’s military victories against the Philistines]; this refrain galled him [literally, “the words were evil in his eyes”]. “They have credited David with tens of thousands [hyperbole for a large number],” he thought, “but me with only thousands. What more can he get but the kingdom [saw David as a threat or a liability rather than as an asset]?”
18:9 And from that time on [marks beginning of Saul’s active opposition against David; perhaps Saul recalled Samuel’s words that God had rejected him as king and would give the kingdom to someone better (cf. 1 Sam. 13:13-14; 15:23,26,28)] Saul kept a jealous eye [looked at David differently than before] on David.