Chukas, a chok, meaning something that we don't understand (a reference to the mitzva of para aduma, the purification from corpse impurity) this is something left over from Hashem to show us how in truth EVERYTHING is purely the Will of Hashem, who is Above All.
The Vilna Gaon explains that geshem (rain) is unique from all forces of nature in that it is the one thing that cannot be understood completely until this very day, exactly how the entire rain process works. He says exactly this point, that Hashem left over one aspect of nature to be above in order to show us that everything is in truth just Hashem. He says that this is the reason why it is called gevuros geshamim (the strength of the rain), for it is the example of Hashem's Rulership. And this is the reason why we find an entire meseches (tractate) devoted to rain (and what is done to merit it)—Meseches Ta'anis.
On this note, the Brisker Rav (see Reshimus haTalmidim) explains what Rashi brings in Parshas Bereishis that Hashem waited to send rain so that Adam haRishon would daven for it—for the whole point of rain is to teach [positive] connection to Hashem (and that is why Eretz Yisrael runs on rain and Divine Connection as opposed to Mitzrayim which has the nilus, the Nile River).
A mitzva that is a chok has seemingly a similar theme, that all aspects of the Torah are all truly just the pure Will of Hashem. And the same thing the adam (person), who is b'tzelem Elokim (in the image of G-d): Don't ever forget how great you are, thanks to Hashem!
"Thank you Hashem!"
"We love you Hashem!"
