Where did the idea that Jesus is the Rose of Sharon come from?
This may have developed from those who see the Song of Solomon as a picture of Christ and the church, though this is certainly not the primary purpose of this book and is really a strong typological use the book. Below are comments from a couple of commentaries that might help, though they approach this differently.
1. I am the rose of Sharon. The bride is still speaking. It is difficult to determine which flower the bride refers to. The only other occurrence of the word in the OT is in Isa 35:1. Crocus appears to be the best translation. Sharon is the Mediterranean coastal plain between Joppa and Caesarea. In the time of Solomon it was a place of great fertility. 2. As the lily among thistles. The bridegroom speaks. In her humility the bride may think of herself only as a beautiful but humble crocus; he regards her as a lily among thistles. So far as lilies surpass thistles, so far does she surpass other maidens. 3. As the apple tree. The bride responds in the same vein. As an apple tree which produces delicious fruit surpasses the other trees of the forest, so does her bridegroom surpass other young men. 4. The king has brought her, a humble country girl, to a banqueting hall. But she need not fear and be bashful in the presence of the young ladies of Jerusalem, for with his love he is protecting her and putting her at ease. (For the thought of protection, see EX 17:15.) 5. Overcome with love and admiration for her lover, the bride asks for raisin cakes (flagons) and apples to strengthen her physically. (Charles F. Pfeiffer, The Wycliffe Bible Commentary, Old Testament, (Chicago: Moody Press) 1962.)
1. Rose—if applied to Jesus Christ, it, with the white lily (lowly, 2Co 8:9), answers to “white and ruddy” (So 5:10). But it is rather the meadow-saffron: the Hebrew means radically a plant with a pungent bulb, inapplicable to the rose. So Syriac. It is of a white and violet color [Maurer, Gesenius, and Weiss]. The bride thus speaks of herself as lowly though lovely, in contrast with the lordly “apple” or citron tree, the bridegroom (So 2:3); so the “lily” is applied to her (So 2:2),
Sharon—(Is 35:1, 2). In North Palestine, between Mount Tabor and Lake Tiberias (1Ch 5:16). Septuagint and Vulgate translate it, “a plain”; though they err in this, the Hebrew Bible not elsewhere favoring it, yet the parallelism to valleys shows that, in the proper name Sharon, there is here a tacit reference to its meaning of lowliness. Beauty, delicacy, and lowliness, are to be in her, as they were in Him (Mt 11:29).
2. Jesus Christ to the Bride (Mt 10:16; Jn 15:19; 1Jn 5:19). Thorns, equivalent to the wicked (2Sa 23:6; Ps 57:4).
2:1. Here the beloved spoke of herself as a rose of Sharon, the fertile coastal region of Israel from Caesarea to Joppa. The Hebrew word for rose is h£a†bas£s£elet. In Isaiah 35:1, its only other occurrence in the Old Testament, it is translated “crocus,” which may be the meaning here. It was a common meadow flower. The lily too was a common flower mentioned often in the Song of Songs (2:1-2, 16; 4:5; 5:13; 6:2-3; 7:2). Though in her humility she likened herself to common flowers of the field, her statement (2:1) reflects a significant contrast with her earlier self-consciousness (1:5-6). Her improvement probably was because of her lover’s praising her (1:9-10, 15).
2:2. The lover echoed his beloved’s newfound sense of worth by comparing her to a lily and all other women to thorns. He agreed that she was a lily (v. 1) but not just any lily! She was as unique among all others as a single lily would be among many thorns. [Walvoord, John F., and Zuck, Roy B., The Bible Knowledge Commentary, (Wheaton, Illinois: Scripture Press Publications, Inc.) 1983, 1985.]