Nine times in this speech of Moses, the Torah mentions fire in the context of what happened on Sinai or Horeb, the two names seem interchangeable. Why so much emphasis on fire? Combine this with the Pillar of Fire that symbolized the presence of God, it seems that fire was a metaphor for the experience the people were meant to have with God. Although the Torah expresses their fear of coming too close and asking Moses to be an intermediary. Which may be why today so many seem to need an intermediary.
Fire is metaphor for God as well as for mysticism. Just think of Elijah’s Chariot of fire or Ezekiel’s first Chapter describing a vision of God. Fire is central. Fire has the capacity to warm and nurture. It can also destroy. It is often used in the Talmud to describe Rabbinic disapproval. Mysticism can enlighten us and it can distort and destroy.
The metaphor of fire for God is an invitation to warm ourselves, to experience the warm closeness of God even if it sometimes comes with pain and discipline (yissurin).
We are so used to the philosophers conceptual, abstract idea of God, detached from experience. The Torah, by emphasizing fire is inviting us to experience the Divine.