In this week's parsha we read in the tochacha, "And if you behave with Me "keri" and you refuse to heed Me, I will add another blow upon you" (Bamidbar 26, 21). Rashi tells us that the word "keri" comes from the same root as the word "mikreh" which means casually. The tochacha comes as a result of behaving casually regarding avodas Hashem: sometimes performing the mitzvos while at other times neglecting them.
Rav Wolbe (Shiurei Chumash) cites the Rambam who offers a different explanation for the word "keri." In the beginning of the halachos pertaining to ta'anios, the Rambam writes as follows: "There is a positive commandment to cry out and blow the trumpets at the advent of any trouble that befalls the populace. . . However, if they do not cry out nor blow the trumpets, rather, they say that what has occurred is due to natural circumstances and this calamity has come merely by chance. . . the calamity will lead to other calamities. In regard to this it is written in the Torah, 'And if you behave with Me "keri" and you refuse to heed Me, I will add another blow upon you'." The Rambam understands "keri" to mean coincidentally - by chance. If one perceives all of Divine Providence as coincidence, he is guilty of behaving toward Hashem in a manner of "keri" and the terrible punishment for such behavior is delineated in the subsequent pesukimof the tochacha.
Additionally, we find that tumah - spiritual impurity - is described as "keri." According to Rashi's explanation, the manifestation of spiritual impurity is the casual performance of avodas Hashem. The Rambam adds another dimension to spiritual impurity: failing to notice the hand of Hashem and instead attributing all occurrences to natural causes.
Both explanations are true. Nothing is by chance. Each and every current event has not occurred by chance; they are carefully orchestrated by the Creator Himself. Additionally, our avodas Hashem should not be performed casually - "by chance." We are His servants at all times and our performance of His mitzvos should reflect that - day and night, rain or shine.