Beis Halleivi on Beshallach

I saw a beautiful בית הלוי today, on פרשת בשלח. I don't have the text in front of me, so let's see how well I can recall it:

כשושנה בין החוחים כן רעיתי בין הבנות -- the Medrosh says that God compares Israel to a שושנה (lily or rose), because at the moment of the Exodus, Israel gave up her idolatrous ways, and immediately started performing mitzvôs, singing praises to God in the שירה. Thus, she smelled fragrant like a rose.

The בית הלוי points out that this is because Israel's naughty behavior had not been due to internal causes, but to environmental causes. (See the Rishônim on the verse שחורה אני ונאוה.) Thus, she was like a fragrant flower, which has been placed near a pile of filth. As soon as you remove the flower from the filth, its fragrance will again clearly fill the air. Similarly, as soon as Israel was removed from the idolatrous environment of Egypt, her fragrant deeds were exuded, strong and clear.

On the other hand, when bad deeds are due to internal causes, the habituation (הרגל) of doing them makes it difficult for one to stop. Thus, in order to set a new pattern of behavior, one must go through a perioud of habituation to the good behavior.

The בית הלוי compares this distinction to a distinction in the Laws of Niddo. If a woman has a וסת קבוע, a set time each month when her period always comes, this וסת is assumed to be halakhically still valid until she has lived through three months without seeing blood on the scheduled day. However, if this וסת קבוע is due not to her internal biological clock, but to some other, external factor (such as eating garlic), then even having a single time (of, say, eating garlic) without seeing blood is enough to annul the halakhic validity of this וסת. Thus, even though an externally-caused וסת can be a וסת קבוע, we treat it as a וסת שאינו קבוע with regard to this rule.

Thus, we see that habituation that is caused by external causes is much easier to break than habituation that is caused by internal causes, because the former does not require a new habituation-period to break the old habituation.