In the shiras hayam, Klal Yisrael praise Hashem for his kindness and mercy. "Nachisa b'chas'dicha am zu ga'alta - With your kindness, You guided this people that You redeemed" (Shemos 15:13.) What kindness are they referring to?
The Midrash (Tanna D'bei Eliahu 23) states that when Klal Yisrael were in Mitzrayim, they gathered together and made a covenant to act toward each other with compassion and sensitivity. What prompted them to make this special arrangement? The Chofetz Chaim (Shemos ibid.) explains that when the Jewish people saw that Pharoh's decrees were becoming progressively harsher, and the oppression was unbearable, they realized they needed a salvation. But what could they do to merit a redemption if they did not deserve to be saved? They decided that if they acted toward each other with kindness and compassion, hopefully that would awaken a heavenly kindness, middah k'negged middah, and Hashem would treat them with mercy and hasten their redemption.
This, suggests the Chofetz Chaim, is the kindness that is hinted to in the shirah. Hashem mercifully calculated the end of the slavery in Mitzrayim in a way that hastened the redemption. As it says in the Haggadah shel Pesach, "Hakadosh Boruch Hu chisheiv es ha'keitz." It was only through divine mercy that Klal Yisrael were given an early release from their persecution in Mitzrayim. But according to the Midrash, the catalyst for that early liberation was the compassion and sensitivity that Klal Yisrael showed toward each other.
The Chofetz Chaim adds that perhaps this is the kindness that Hashem highlights, in the words of Yirmiyahu HaNavi (2:2), "Zacharti lach chessed ne'urayich - I remember the kindness of your youth." Hashem is saying that He will never forget the chessed that Klal Yisrael did with each other in Mitzrayim. And in fact, it was that kindness which tipped the scales and brought about yetzias Mitzrayim.
This idea that human kindness can generate divine mercy is applicable not only to the collective unit of Klal Yisrael, but to individuals as well. Chazal comment in different places that one who acts with compassion is judged more favorably. "One who is merciful with others is shown divine mercy" (Shabbos 151b.) "One who relinquishes his measure of retribution (ma'avir al midosav), the heavenly tribunal overlooks all of his sins" (Yoma 87b.) "Whose transgression does Hashem pardon? One who overlooks sin committed against him" (Rosh Hashana 17a.)
The Ramchal (Mesilas Yesharim 19) explains that by strict legal standards, no one would receive a favorable heavenly judgement. But by acting with kindness and sensitivity toward others, a person can merit a merciful judgement because even if Hashem follows the strict rule of law, one who acts with kindness should be judged with the same measure of kindness. Therefore, the more compassionate a person is with others, the more compassion he will be shown in the heavenly court.
In the tefillah of selichos on fast days and especially on Yom Kippur, we invoke Hashem's thirteen attributes of mercy multiple times. Chazal understood (Rosh Hashana 17b) that Hashem promised Moshe Rabbeinu that these thirteen attributes of mercy will never return empty-handed - einan chozros reikam. Rashi writes that what Hashem meant is that whenever Klal Yisrael mention these attributes of mercy, their tefillah will always be accepted. But this seems difficult because unfortunately there are times that our tefillos are not answered favorably despite the fact that we mention the thirteen attributes of mercy.
Rav Avraham Saba explains in Tzror Hamor (Shemos 34:5) that Hashem did not mean that simply mentioning the attributes of mercy will cause the tefillos of Klal Yisrael to be answered. Rather, if Klal Yisrael exemplify these attributes in their behavior, "to show mercy, to be gracious to the weak, to be slow to anger, to act with kindness one to another, to overlook injustices committed against them", then their tefillos will never be ignored. When Hashem told Moshe, "Ya'asu lefanai k'seder hazeh - Let them perform this order before me," He did not simply mean that they should recite this order of attributes. What He meant is that their behavior should reflect these attributes, and that will ensure that their tefillos will always be answered favorably.
In times of crisis and challenge, we need divine mercy to pull through. The way to earn Hashem's kindness is by acting with compassion and sensitivity toward others. When we incorporate the thirteen attributes of mercy into our behavior, we are assured that middah k'negged middah, Hashem will act toward us with mercy as well.