What are the connotations of "Lo Sosifu ... , ve'Lo Sigre'u mimenu"?
Rashi and Ramban #1 (citing the Sifri): 'For example, not to write five Parshiyos in one's Tefilin, not to take five species with (including) the Lulav or to tie five Tzitziyos on (the five corners of) one's garment'; 1 and likewise "ve'Lo Sigre'u"
Ramban: Or even to add some words to Birchas Kohanim - such as "Hashem Elokei Avoseichem Yosef aleichem Elef Pe'amim".See Torah Temimah, note 1.
Ramban: See Rosh Hashanah, 28b.
Ramban: Like Yerav'am ben N'vat - See Melachim 1, 12:33 - Megilah, 14a and Yerushalmi, Megilah, 1:7. See also Ramban, DH 'u'Lefi Da'ati' who concludes that we learn the prohibition against a Navi adding Mitzvos from "Eileh ha'Mitzvos" (Behukosai Vayikra, 27:34), and that the Takanos that the Chachamim instituted - such as Sheniyos la'Arayos, are actually min ha'Torah which we learn from the Pasuk in Acharei-Mos Vayikra, 18:30, "u'Shemartem es Mishmarti").
See also Ba'al ha'Turim. Refer also to 4:2:152:1*.
Why does the Torah add the (otherwise superfluous) word "mi'Kirbecha"?
Oznayim la'Torah: Because Hashem only destroyed all thoe from Yisrael who worshipped Ba'al Pe'or, but not Nochrim from among the nations. 1
Oznayim la'Torah: In fact, Nochrim are even permitted to serve Avodah Zarah, provided they serve Hashem as well. Not so Yisrael, who are obligated to serve Hashem alone ("Ein Od mi'Levado"), and anyone who serves idols where there are no witnesses, Hashem destroys in order to prevent idolatry from speading among the people.
What can be wrong with adding to Mitzvos?
Da'as Zekenim, Hadar Zekenim and Rosh: When one adds, one detracts. If one put five Tzitziyos, one detracts from the four, since one did not do the Mitzvah as per instructions. The same applies to five Parshiyos in one's Tefilin and the five species of the Lulav. However, if one sits in the Sukah for eight days, the eighth day does not come with the seven (to invalidate them, since at the time one performed the Mitzvah corectly), and what one did is done.
Because if people will be allowed to add Mitzvos, they will assume that, by the same token, they will be allowed to subtract Mitzvos too.
Hashem's Mitvos are perfect, and any change is bound to detract from that perfection. 1
See also Torah Temimah, who elaborates at length.
Why does the Torah repeat in Re'ei, 13:1 "Lo Sosef alav ve'Lo Sigra mimenu"?
Refer to 13:1:1:1.
