The Song of Songs describes the love between a young man and woman before their wedding – and even before their engagement. Of course, the actualization of that love will come only after their wedding, according to the laws of the Torah. From that point on, the love between them will intensify, with God’s help. But even before all those stages, a basic, natural affinity exists between the couple, which the Torah comes to establish and sanctify.
That initial affinity could be ‘surrounding light,’ in which others identify that a certain young man and young woman could be a potential match. This affinity could determine the fact that this couple will marry. It could also be ‘internal light’, an affinity that flows from the man and woman’s inner feeling that they simply like each other. These two types of affinity appear in the marriage of Jacob, “the choice of the Forefathers” to Rachel “the mainstay of the home,” in whose merit Jacob fathers the 12 tribes of Israel. From the time that she was very young, people would say that Rachel, the younger of Lavan’s daughters, was destined for Jacob, the younger of Rebecca (Lavan’s sister)’s son. From the moment that Jacob and Rachel met, it was love at first sight (which preceded their actual marriage by more than seven years).
In Jewish law, affinity describes the connection between a couple who is supposed to marry in the framework of yibbum (Yibbum is the form of levirate marriage found in Judaism. As specified by Deuteronomy 25:5–10, the brother of a man who died without children is permitted and encouraged to marry his brother's widow) prior to the actual yibbum (or the procedure of chalitza, in which the affinity is nullified). Abstractly, this affinity describes the secret connection that exists between every bride and groom before they wed and actualize their love. It is specifically in the case of yibbum, in which the connection has been mandated by Heaven – in tragic and inexplicable circumstances – that the fact that the affinity between the two is above reason and understanding is highlighted.
Spiritually, this connection also appears in Jacob’s construction of the House of Israel. Leah, who was destined for Esau (the older daughter for the older son) rejected marriage to such an evil person. Since “an evil person is considered dead” her marriage to Jacob can be considered yibbum (and she even married him without his prior knowledge, in the manner of yibbum without prior knowledge).
Why is it important to emphasize that the connection between a man and a woman, legally binding by the Torah – is based on a pre-existing natural affinity? One of the foundations of natural morality common to all mankind is faithfulness, expressed in the safeguarding of the covenant between husband and wife. Hence, faithfulness is associated with the attribute of yesod (foundation) – the power to forge a covenant. By definition, a covenant is a connection that is beyond reason – the commitment of the couple to preserve the bond of love between them – even if the logical reasons for doing so become blurred as life goes on. This loyalty can be built while the connection between them is completely natural, also above and beyond the reason of the Torah. It is forged while the man and woman are still free to decide to end their relationship and look elsewhere – but choose nonetheless to remain loyal to each other until they actualize their love by means of marriage according to the Torah. This natural faithfulness, which flows from the pre-wedding affinity, remains the eternal bulwark of the marriage, sanctified by the Torah.
 Nedarim 64b.