Why does it say "great waters cannot extinguish the love"?
Torah Temimah citing Sotah 21a: A sin can inhibit the reward for a Mitzvah, but not the reward for Torah. 1
Rashi: Because the love was called flames (6), it is appropriate to say "cannot extinguish." "Great waters" are the nations.
Malbim (Melitzah): All enticements of the Yetzer and desires of the power of arousal cannot nullify the love of Nefesh Shlomo for her G-d.
Malbim (Mashal): All Shlomo's strategies to veer her heart from her beloved did not help at all.
Orchos Tzadikim (Sha'ar Leshon ha
What is the meaning of "u'Nharos Lo Yishtefuha"?
Rashi: The officers and kings of the nations cannot flood (overcome) the love via force, threats, or even enticement.
Malbim (Melitzah): Upheavals of the time and worldly evils cannot overcome the love.
Malbim (Mashal): Normally, rivers inundate everything in their path. They cannot annul this love.
What do we learn from "if a man would give all his wealth for love, they will scorn him"?
Torah Temimah citing Sotah 21a: Hillel and Shevna were brothers; Hillel engaged in Torah, and Shevna in business. Shevna said, let us be partners, and split the rewards (in this and the next worlds). A voice from Heaven said "Im Yiten Ish... 1 "
Rashi: If one would give all his wealth to make you (Yisrael) switch your love, they would scorn him. Hashem and His Beis Din testify all this about Yisrael's love. Seforno - this is because Yisrael do not value anything equal to Ahavas Hashem.
Malbim (Melitzah): If one would give all his wealth for Ahavas Hashem, they will scorn him, for it is not acquired for a small amount - only for Mesiras Nefesh and [all] his money. Nefesh Shlomo clings permanently to Elokei ha'Ruchos; this completes the fifth Shir.
Magen Avos (1:14): Shevna is scorned, for he did not offer to support Hillel before he learned. R. Yerucham (Toldos Adam v'Chavah 2:5) - if one tried to sell the reward for what he already learned, he loses it (and will be scorned). Maharsha - Shimon Achi Azaryah and R. Yochanan were scorned in this world (they were called by the name of their supporters) and the next (they lost half the reward for their learning). One should not do like them. Rather, he should refuse, like Hillel.