In two places in the Torah, Hashem gives Bnai Yisrael a long series of curses should they stray from Him. One set is here in Parshat Bechukotai, and the other is in Parshat Ki Tavo. In both places the curses are extremely harsh and severe. They are so severe, that when they are read in Shul, they are usually said in an undertone because we do not want to have to listen to them. There are, however, several differences between the set of curses in the Tochacha of Sefer Vayikra and that of Sefer Devarim.
Chazal, on this topic, state in the Gemara in Baba Batra (88b) that there are more curses in Devarim (98) than in Vayikra (49). However, in a different Gemara (Megillah 31b) it says that the second set of curses is milder than the first in two ways. Firstly, the curses in Devarim are addressed to the individual and the curses in Vayikra are addressed to the community as a whole. Secondly, the first curses were stated by Hashem Himself and the second were stated by Moshe.
The Maharsha, in his commentary to the Gemara, says that the second reason for the milder version of the curses in Devarim should not be understood at face value. The first set of curses was said at a point in Jewish history when the Jews were not yet responsible for each other. The second set, however, were said right before the Jews entered Eretz Yisrael, and at this point, all the Jews were responsible for each other. This interpretation explains the difference as only showing a change in the attitude of Bnai Yisrael, not showing a change in the curses themselves.
Ramban, after taking the position that the first set of curses refers to the destruction of the first Beit Hamikdash, and the second set of curses refer to the destruction of the second Beit Hamikdash, explains the difference between the items said by God and those said by Moshe. In the first Beit Hamikdash, says Ramban, all miracles were performed openly, for all to witness, thus revealing the Shechina of Hashem. This is the reason that Hashem says the first set of curses. In the second Beit Hamikdash, however, there were no open miracles, meaning God did not reveal his Shechina. This is the reason that Moshe said the second set of miracles.
Another difference between the two sets of curses is the structure in which the curses were given. The first set was given in stages. This comes to imply that if Bnai Yisrael repent at any given stage, the following stages will not come. The second set, however, were all given together, implying that all the curses would be delivered at once, without breaks to allow for repentance.
We must strive to reach a level in which these curses no longer apply to us and we only receive the blessings that are the counterpart to the curses.