This is the aspect of “ל וִי-Levi," which comes from the phrase that Leah said when naming him “now, my husband ‘יִל ָּוֶהַׁ-will join’ with me” (Bereishis 29:34).
The word “לֵוִּי” means to accompany or to connect to someone. Leah said that because she gave birth to a third son, Levi, now her husband to connect to her even more than before. (This was connected to the fact she gave birth to her full quota of children, since Yaakov had four wives and was destined to have twelve children, so each wife’s share was (theoretically) three children each.)
This represents a special connection to Hashem that comes after we have the first two spiritual “children” of love of Hashem (Reuven) and fear of Hashem (Shimon). Once we attain these two aspects, we can properly “connect” to Hashem in Torah study.
This is like the idea that we say in the Shabbos night meal song: “Shabbos is surrounded by weekdays on both sides, like a bride standing between people on her right and left.”
Shabbos is compared to a bride. Just as a bride is accompanied by two people, one on her right and one on her left, escorting her to the Chuppa (usually her mother and future mother-in-law, or her mother and father), so too, Shabbos is surrounded by weekdays before and after it, preparing for Shabbos and extending the influence of Shabbos.
Similarly, in order to become a “bride” to Hashem, we need to be accompanied by two aspects of love of Hashem (Reuven) and fear of Hashem (Shimon). The actually “marriage” that happens will be with Torah study, as will be further explained.
