Or HaChaim on Deuteronomy 11:21/אור החיים על דברים י״א:כ״א

1 למען ירבו ימיכם, "that your days may multiply, etc." Why did Moses not write this verse prior to verse 20, i.e. the writing down of this paragraph and attaching it to the doorposts? If that had been the case our verse and the blessing it contains would also be part of the promise in what was to be attached to the doorposts? Perhaps Moses chose this order in order to teach us that the fulfilment of the commandment in verse 20 would bring in its wake the blessing contained in verse 21. This may be what the Talmud (Shabbat 32) had in mind when it stated that "anyone who takes the commandment of the Mezuzah seriously has his lifespan extended." [The only quotation I was able to find on that folio was that neglect of the commandment of Mezuzah may result in premature death. This itself is questionable. Ed.]