"If you will go in [the way of] My laws" (Vayikra 26:3), the opening phrase of parshas Bechukosai, is interpreted by Rashi, "she't'hi'yu ameilim baTorah - that you should be laboring in the Torah". Conversely, Rashi explains the opening phrase of the curses, "If you will not listen to Me" (26:14), as referring to not laboring in the Torah.
The pivotal and critical distinction between the behavior of Am Yisrael which deserves blessing and the behavior which results in the horrific curses of the tochacha is whether we are ameilim baTorah or not. If we do labor in Torah, and, as a result, "observe My commandments and perform them" (26:3), we are blessed with bounty, peace, victory, fertility, and the spiritual rewards of the Bais Hamikdash and Gan Eden (26:4-12).
The causes of the curses, according to Rashi (16:14), are "seven sins, the first brings on the second, and so forth until the seventh. They are the following: He did not study [i.e. labor intensely in the Torah], he did not perform [the mitzvos], he despises others who perform [the mitzvos], he hates the [Torah] scholars, he prevents others [from performing mitzvos], he denies the mitzvos, he denies Hashem". This progression, from bitul Torah to k'fira, from a failure to labor in Torah to outright atheism, has tragic consequences. But how does one lead to the other?
Rav Eliyahu Dessler (Michtav Me'Eliyahu vol. 3, pg. 177) provides a crucial insight: inner faith comes as a result of learning Torah in depth, not by abstract speculation and philosophy. Human reason is "bribed" by all types of personal interests (negi'os); desire leads reason to wherever it wishes. Relying on one's own independent human reason is comparable to someone going to a judge that he bribed in order that the judge will rule for him as he wishes. As such, one who says "I will only accept what I understand" can never apprehend the truth because he is swayed by his desires. Instead of building on our own subjective and limited human reason, our faith must be firmly rooted in the Torah tradition (mesorah) received from previous generations, and on learning Torah and recognizing its greatness and the greatness of our Sages. Only one who is rooted in, and subservient to, the Torah, as explained by mesorah, can attempt to apprehend the fundamentals of faith rationally.
This insight of Rav Dessler explains how a failure to learn Torah in depth can lead to heresy. Laboring in the Torah for its own sake yields clarity that Hashem gave the Torah at Sinai, and joy similar to when Torah was given at Sinai (ibid pg. 176, based on Talmud Yerushalmi, Chagiga 2:1).
"Do not stray after your hearts" (Bamidbar 15:39) refers to heresy (Berachos 12b). Hashem implanted within man the quality of curiosity in order to drive us towards deep learning of Torah. However, we may not be curious to learn about heresy. Our evil inclination misuses our curiosity to lead us to heresy, and even idolatry, so that sexual immorality is permitted publically (Sanhedrin 63b). To overcome this, one should reinforce one's simple faith based on tradition, and labor incessantly in Torah (pg. 178-9).
Today we are witness to an exponential increase in the labor of Torah, in numbers unprecedented in the post-Talmudic era (see Rambam, introduction to Mishne Torah). We must constantly thank Hashem for this phenomenon, and the attendant blessings, relative to our recent past, of bounty, peace, victory, and fertility.
At the same time, sadly, unprecedented numbers of Jews are being lost to assimilation and intermarriage, as the progression of the seven sins highlighted by Rashi continues to play out before our eyes. Even affiliated Jews, including even some who identify as Orthodox, are involved in some of the negative actions and attitudes which are listed amongst those sins, and are progressing down the destructive path towards assimilation and the concomitant curses. Simple, unquestioning emunah (faith) in the eternity and morality of the Torah is being derided, even among observant Jews, by proponents of the postmodern zeitgeist described and anticipated by Chazal in Sanhedrin (63b) and by Rav Dessler.
It remains for the faithful to strengthen our faith and labor in Torah. May we thereby merit the continued and enhanced worldly brachos, and, ultimately, the eschatological ones as well.