The Rebbe of Blendov zy”a (Sefer Emes L’Yaakov) explains this pasuk by saying that when some people are struck with an illness or any form of difficulty, they consider it “natural”, rather than recognizing that it is a result of their sins. They say that “the same thing could happen to anyone.”
The Torah is hinting to this when it says not to be “sakiru panim b’mishpat.” The word “sakiru” has a root of “nachri” (stranger). Thus, the pasuk is saying that if someone finds himself in a situation of mishpat (harsh judgment), he should not say that the same thing could happen to any stranger, and it is only by happenstance that it happened to him. Rather, one should recognize that Hashem sent this form of suffering specifically for him. As the Gemara says (Brachos 5B), when one faces a difficulty, he should investigate his actions.
