Jamieson, Fausset and Brown on Song of Solomon 8:6

6. Implying approaching absence of the Bridegroom.

seal--having her name and likeness engraven on it. His Holy Priesthood also in heaven (Ex 28:6-12, 15-30; He 4:14); "his heart" there answering to "thine heart" here, and "two shoulders" to "arm." (Compare Jr 22:24, with Hag 2:23). But the Holy Ghost (Ep 1:13, 14). As in So 8:5, she was "leaning" on Him, that is, her arm on His arm, her head on His bosom; so she prays now that before they part, her impression may be engraven both on His heart and His arm, answering to His love and His power (Psa 77:15; see Ge 38:18; Is 62:3).

love is strong as death--(Ac 21:13; Ro 8:35-39; Re 12:11). This their love unto death flows from His (Joh 10:15; 15:13).

jealousy ... the grave--Zealous love, jealous of all that would come between the soul and Jesus Christ (1Ki 19:10; Psa 106:30, 31; Lu 9:60; 14:26; 1Co 16:22).

cruel--rather, "unyielding" hard, as the grave will not let go those whom it once holds (Joh 10:28).

a most vehement flame--literally, "the fire-flame of Jehovah" (Psa 80:16; Is 6:6). Nowhere else is God's name found in the Song. The zeal that burnt in Jesus Christ (Psa 69:9; Lu 12:49, 50) kindled in His followers (Ac 2:3; Ro 15:30; Php 2:17).