Parashas Metzora · Passuk 15:19
Chabad.org — English translation + Rashi
Torah Temima points to Niddah 31b.
English: “Seven days she shall be in her niddah” — it was taught: R’ Meir used to say: Why did the Torah say niddah for seven [days]? Because he is accustomed to her and becomes disgusted with her; the Torah said she shall be impure seven days so that she will be as beloved to her husband as at the time of her entry under the chuppah.
Moral reason for seven-day niddah
Chazal offer a relational rationale for the seven-day niddah period: periodic separation renews affection.