It is written in the Zohar, "My children, [I swear] by your lives that there is nothing closer to Hashem than a person's heart, and He is happier with it more than all sacrifices in the world." One's heart is the "holy of holies" of his spiritual makeup says Rav Wolbe (Alei Shur vol. II pg. 65). If a person would direct his desires and thoughts towards his Creator, Hashem would be closer to him than anything else in the world. Being that the heart is the location of the closeness, within the heart lies a natural yearning towards the Creator.
This idea is mentioned by Rashi in this week's parsha. "And if you behave with me "keri" and you do not listen to me, I will continue to smite you seven ways like your sins" (Vayikra 26, 21). Rashi cites Menachem who explains that the word "keri" can be translated as refraining. Rashi elaborates, that this is similar to the Aramaic translation offered by Onkelos - with hardness - because "they harden their hearts to refrain from coming close to Hashem." Rav Wolbe notes that it is within the very nature of the heart to strive for closeness to Hashem. Only if one deliberately chooses to harden his heart will he succeed in silencing this inherent drive!
This idea runs contrary to popular belief. Most people are of the opinion that yearning and longing for Hashem are solely the lot of spiritually lofty people. However, this is not the case. The basic nature of the heart is to crave Hashem's closeness. The most fundamental sin is to suppress this innate yearning of the heart, and it is around this aveirah that the entire tochacha revolves.
With this we can understand the punishment that follows in its wake. "And I will incite the animals of the field against you..." Rashi explains that not only will wild animals attack as is their nature; even domestic animals will act out of character and attack people. Rav Wolbe explains that this punishment is meted out measure - for - measure. The people went against their grain and suppressed their natural yearning towards Hashem, and in turn Hashem caused the normally domestic animals to go against their nature and attack the evildoers.
When can one feel this closeness to Hashem that the heart desires? One can feel it during tefilla. It could very well be that our tefillos are "dry" because we do not have a true desire to come close to Hashem. We have in effect "hardened our hearts to refrain from coming closer to Hashem." Our first step is to truly want a relationship with our Creator, and then, through our tefillos we will strengthen our natural inclination for kirvah and bring ourselves closer to Hashem.