Question:
Mishna Berurah (1:2) says the Shlah says to connect day & night with tefillah and llimud hatorah.
Mishna Berurah (1:11) says the Shlah says to say ‘al naharos bavel’ before bentching.
Why is the latter considered Halacha and the first is generally considered to be a “good thing to do” but not necessarily an obligation?
Answer:
Interesting question. The reason may be because the Shlah is telling us an idea based on Sod, (see M:B there) or kabalah, which is meant for the pious, and is not as basic as things that are coming from a halachic backround, such as mournig over the churban Bais Hamikdash. Therefore the minhag to say al naharos bavel, is more of an obligation.
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