[Moses told the Jewish people,] "G-d, our G-d, spoke to us at Horeb saying, 'You have dwelt too long at this mountain.'".
G-d here is alluding to the lesson that we should never remain too long on the same level in our relationship with Him, without advancing or ascending.
This idea is also articulated in the Prophets, where the human potential to progress in Divine consciousness is contrasted with the angels' lack of this potential: "If you go in my ways....I will make you into those who walk [i.e, constantly move forward] in contrast to these [angels], who [merely] stand here."
Angels, being personified emotional states of involvement with G-d, are static, whereas human beings can progress from one level to another in their emotional involvement with G-d. In fact, we should strive to reach the next level of spirituality as soon as we become aware of its existence.
Furthermore, this verse teaches us not to cloister ourselves in the study hall, devoting ourselves exclusively to our own self-refinement. Rather, G-d challenges us to leave this pristine and holy environment, traveling to a place far from "His mountain," to illuminate even these distant places with the Divine light of the Torah.
Daily Wisdom #2