1) The revelation at Mount Sinai transformed the Israelites, restoring them to the purity of Adam and Eve before their sin. However, the rest of the world was not transformed in the same way. 2) As a result, while the soul of Israel received the Torah, their "body" - the rest of the world - did not. Israel and the world are interdependent, so Israel could only rise spiritually if the world also rose. 3) On Shavuot, we commemorate becoming a "priestly kingdom" tasked with preparing the world to deserve redemption. We must pray for and help raise the spiritual level of the whole world.