The Mishnah in Megillah 31a states that one may not divide the tochacha of Parshas Bechukosai into more than one aliyah. This is codified in Shulchan Aruch (Orach Chaim,428:6).
The Gemara in Megillah 31b gives two reasons for this rule. One is based on Mishlei 3:11, “Mussar Hashem, b’ni al timas – My son, don’t be disgusted with Hashem’s mussar.” Rashi explains that if we divide the tochacha, it will appear as if we stopped in the middle because the first oleh was disgusted with it.
The second reason is offered by Reish Lakish who explains that one should not recite a berachah on puraniyos (punishments). Therefore, we begin the aliyah a few pesukim before the tochacha and don’t stop reading until a few pesukim after it is completed. If we would separate it, the second oleh would invariably have to recite a berachah on puraniyos.
The Gemara states that the halacha not to break up the tochacha only applies to the tochacha in Vayikra, not to the tochacha in Devarim. The Gemara’s distinction: the tochacha in Vayikra was addressed to Israel in the plural and said by Moshe in the name of Hashem, while the tochacha in Devarim was addressed to Israel in the singular and said by Moshe in his own name.
The Rambam (Hilchos Tefillah,13) codifies this rule in halacha seven. In halacha five the Rambam writes that one should always make sure to begin and end with a positive subject. It is apparent from the fact that the Rambam wrote these two halachos separately that the halacha to begin and end with something positive is distinct from the halacha not to divide the tochacha.
The Rema (Orach Chaim, 138) writes that one should always make certain that the reading of the Torah begins and ends with something positive. However, the Rama cites the halacha in 428:6 – that one may not divide the tochacha – as the reference for this rule. Apparently the Rema is following Reish Lakish’s explanation that the reason we don’t the tochacha is because we shouldn’t make a berachah on puraniyos – in other words, the rules about the tochacha and beginning and ending with a positive subject are linked.
And yet, the Gemara in Megillah clearly states that the halacha about the tochacha only applies to the tochacha in Vayikra, not to the tochacha in Devarim. This despite the fact that both tochachos are full of dire punishments. Perhaps we can say that Reish Lakish’s rule that we shouldn’t make a berachah on “puraniyos” only refers to the severest punishments (the ones in Devarim, we explained, are slightly less harsh), but the Rema clearly writes that we should always begin with something good. The tochacha is clearly not good (in the ordinary sense of the word). What then is the source of his statement that one must always begin with something positive?
The Vilna Gaon, on the Rama, cites a Yerushalmi that seemingly disagrees with the Bavli, saying that an additional reason why we should not divide the tochacha is because we should always begin with something positive.
The sefer Hararei Kedem suggests that the halacha not to divide the tochacha in Vayikra is m’akeiv. In other words, if a person does divide it, he must read it again. If he doesn’t, he will not have fulfilled his obligation. On the other hand, the halacha to begin and end with something positive is not m’akeiv.
That’s why the Rambam writes that one may divide the tochacha in Devarim – for this is the ikar halacha. However, based on the rule that one should always begin with something positive, the Rambam says that the minhag is not to divide either tochacha.