In this week's parsha the Torah juxtaposes the mitzvah of sending away the mother bird before taking her offspring and the mitzvah to erect a fence on one's roof upon building a new house. Rashi explains that if one performs the mitzvah of shiluach hakein he will merit building a house and performing the mitzvah of erecting a fence, because "one mitzvah leads to another mitzvah" (Avos 4, 2). Rashi continues that it is for this very reason that the Torah places the prohibitions of planting a field with kelayim (a forbidden mixture of seeds) and the prohibition of wearing clothing that contain shatnez (a mixture of wool and linen), directly after the above mentioned mitzvos. Performance of the original mitzvos will lead to the acquisition of a field, a vineyard and clothing, and to the performance of the related mitzvos.
Rav Wolbe (Shiurei Chumash) points out that we generally understand that the Mishna, "One mitzvah leads to another mitzvah" is limited to the actual performance of the mitzvah: The performance of one mitzvah will give me Heavenly assistance to perform another mitzvah. If I say bircas hamazon I will be able to learn in the morning and thereafter conduct my business honestly and daven mincha with a minyan, etc. However, from Rashi it is clear, that additionally, through the performance of one mitzvah Hashem creates and allocates the resources needed to enable the performance of mitzvos that hitherto had been entirely out of the person's ballpark. If he does shiluach hakein Hashem will give him the ability to build a house enabling him to perform the mitzvah of building a fence. Truly amazing!
There is yet another aspect of this adage of Chazal. Rav Wolbe (Alei Shur vol. II pg. 40) quotes Rav Dessler who explains that every person has a "nekudas habechira." In other words, there are many mitzvos and good deeds that a person does without choosing at all; rather, they are dictated by the way he was brought up or his intrinsic nature. Likewise, there are many aveiros that he does without even realizing that they are wrong; it is simply the way he was educated. His nekudas habechira (point of choice based on his free will) is merely at one specific location: where what he knows to be true clashes with what he imagines is true (but deep down really knows it's not).
However, one's nekudas habechira is not stagnant. "One mitzvah leads to another mitzvah" is in effect a conditioning agent whereby a person becomes acclimated to the performance of a mitzvah to the point that the yetzer hara no longer tries to persuade him to disregard that mitzvah because it has ceased to be a challenge to overcome the temptation. He can now move up to the next rung on the spiritual ladder and conquer even bigger and better territories as he advances in his avodas Hashem.
If a mitzvah presents itself, grab the opportunity. Besides the infinite reward of that specific mitzvah, it also has the ability to improve one's spiritual and physical standing. Moreover, in the month of Elul each mitzvah is precious and might be the one to tip the scales!