According to Rava (and fitting into the first asmachta), the reason for eruv tavshilin is to honor the shabbos by making sure we don't finish eveything up on yom-tov and neglect the shabbos.
It comes out that: m'deoraisa, we are allowed to prepare from yom-tov to shabbos, but d'rabanan, we must first prepare an eruv tavshilin to ensure that we will not wind up neglecting the shabbos. As a result, if we don't prepare the eruv, then we cannot cook for shabbos on yom-tov. Doesn't this seem a little self-defeating? If the whole concern is for shabbos, then why restrict one's ability to cook for shabbos on yom-tov at all? If one forgets to make the eruv, he no longer has the ability to cook on yom tov and to make sure that he has enough ready for shabbos! In effect, the eruv is taking away from kavod shabbos!
Thank you for your time.
Avi Hoffman, Hillside, NJ, USA
That is indeed true. An effect of the Gezeirah here is that it causes a person to lose exactly what we had wanted him to gain through the Gezeirah. However, the Rabanan decided that it is worthwhile for a person to lose one experience of Kavod Shabbos, in order to learn a lesson that will cause him to enjoy many experiences of Kavod Shabbos in the future. (See Yoma 85b, "Transgress a single Shabbos in order to fulfill many future Shabbosos."). It is somewhat like allowing a child to burn himself with a small, controlled, flame, in order that he may learn to keep away from fires in the future.
We find similar concepts in numerous instances in the Talmud.
For example, it is prohibited to shave during the Regel so that a person should not enter the Regel unshaven, but should make sure to shave beforehand. If he doesn't shave beforehand, though, then he must remain unshaven throughout the Regel.
Also, according to Rabeinu Yonah (Berachos 9a), it is prohibited to say Keri'as Shema after Chatzos so that a person will take to heart to read Keri'as Shema early in the evening and not push off Keri'as Shema until the late night, when he might fall asleep and miss Keri'as Shema altogether. However, if he does not read Keri'as Shema before Chatzos, the Rabanan tell him that he must miss Keri'as Shema entirely, and he may not be Yotzei by reading it after Chatzos (although mid'Oraisa he should be Yotzei the Mitzvah by doing so). See also Tosfos Sukah 3a DH d'Amar, who proposes a similar Halachah regarding sitting in a Sukah.