"Rebbi Tarfon says, 'The day is short, the work is great... and the Master of the house is pressing" (Avos 2:20). The Yavetz explains that "the Master is pressing" refers to Hashem who desires to bestow upon us goodness, for it was to this end that He created the world. What does this mean? Rav Yeruchom Levovitz cites the Seforno (Shemos 12:42) who explains the Torah's reference to the night of redemption from Egypt as "leil shimurim." Hashem wished to redeem Yisrael but He did not find them spiritually worthy until that night, for which He had been waiting (mishamer) because He desires to perform kindness. In a similar vein, we must try to accomplish as much as possible since Hashem (the Master of the house) is pressing and waiting for the moment that He will be able to bestow His infinite kindness upon an individual and reward him for the spiritual level that he achieves.
Rav Wolbe (Daas Shlomo) quotes the Seforim Hakedoshim who write that when a person is born he is allotted the amount of days necessary to perfect himself. Therefore, one must take advantage of each day because today's mission cannot be accomplished tomorrow. A day which was not utilized will ultimately manifest itself as a "hole" in a person's perfection. Every day carries with it new spiritual opportunities, and it was with this in mind that Moshe Rabbeinu davened, "Teach us to count our days" (Tehillim 90:12).
The Zohar makes a statement similar to the above mishna in Avos. "The days are short and the baal chov is pressing." However, the meforshim explain that the baal chov mentioned refers not to Hashem but to the yetzer hara. Every day brings with it new challenges as Chazal tell us, "A person's yetzer hara renews itself every day" (Kiddushin 30b). Every individual is allotted a "consignment" of yetzer hara that he is meant to rectify over the course of his life, and we were provided with 613 mitzvos to be used as weaponry to combat the yetzer hara. This too was included in Moshe Rabbeinu's tefillah "teach us to count our days;" i.e. help us be alert to the daily trials presented by the yetzer hara.
Both explanations of Moshe's tefillah, continues Rav Wolbe, are very pertinent to the days of Sefiras Ha'omer where we literally count each day. Teach us to count each day and maximize it to its fullest potential. Moreover, we are taught that these days are replete with a bounty of Heavenly assistance in attaining spiritual acquisitions. On the night of Yetzias Mitzrayim, Hashem elevated Bnei Yisrael to the apex of spiritual ascension; a level that they certainly had not achieved on their own. They were given a brief taste of perfection and then they spent the next forty nine days toiling to regain this elevated plateau. The siyata d'Shmaya that they experienced during that era which enabled them to make a spiritual turnaround at a dazzling speed, is reawakened each year during the days leading up to Shavuos.
The Alter of Kelm writes that each day of the sefira, Bnei Yisrael acquired one of the forty eight qualities through which the Torah is acquired (see Pirkei Avos 6:6). Rav Wolbe concludes that while the Alter himself certainly worked on a new quality each day, it is more than enough for us to work on a single quality over the course of the entire sefira. Any quality that assists one in acquiring Torah knowledge is a vital asset. Each quality brings a greater connection to the Torah which not only aids a person in using every day to the fullest, it is also gives a person more ammunition against the yetzer hara who assaults us with new challenges on a daily basis.