Holy Things Come in Groups of Seven
Torah learning
The Maharsha explains that in this world all holy things come in groups of seven, corresponding to the seven days of Creation. Because the world was created in seven days, holy things come in sets of seven, and this is why the harp of the Bais HaMikdosh had seven strings. The sefer Ohev Yisroel touches on this as well in Parshas Behaaloscha, and explains it on the basis of Kabbalah.
Source: Maharsha, Chiddushei Aggodos, Arachin 13b, s.v. Kinor; Ohev Yisroel, Parshas Behaaloscha, s.v. kach es haLevi'im