Why does every mitzvah a Jew performs activate great and awesome spiritual consequences in the upper worlds? 

Because Hashem commanded us to do mitzvos—we are His agents! And the Gemara (Nedarim 72b) teaches that an agent is like the one who sent him. 

Thus, when we do a mitzvah, it is Hashem acting through us—no wonder it has such an immense impact.

~ Based on the Shem MiShmuel