- Question: Why is saying Shema with the congregation derech eretz, a cordial thing to do even if you are not praying with them but there is an obligation to say Kedusha with the congregation even if you are not praying with them?
Background: A. The Shulchan Aruch (Orach Chaim 65:2 and Mishna Berura 9) says that if you already read the Shema and you enter the shul and find them reading the Shema you must read the first verse so that you don’t look like as if you don’t want to accept the yoke of Heaven with your friends. This applies to other things that the congregation says together, for example “Ashrei” or “Aleinu,” you should read with them because it is derech eretz, proper manners. B. The Rema (125:2) says there is an obligation to say kedusha with the congregation and Rav Moshe Feinstein (Igros Moshe Orach Chaim 3:89) says that this is a halachic obligation, not just proper manners. C. There is a set obligation to say the Shema twice a day, once in the morning and once at night. D. The concept of saying Kedusha is to sanctify the name of Hashem within a congregation.
Answer: Since the obligation is to say Shema twice at some point in the day and at night then to say it when everyone else is saying it and you already said it is only proper manners, derech eretz, to not look like you don’t want to accept the yoke of Heaven. But to sanctify Hashem’s name with the congregation potentially really could be even a hundred times a day, there is no limit, therefore every opportunity is a real obligation.