Maybe it symbolizes the fire in our souls that is lit when we serve God. It is a unique fire - a fire that doesn't consume - and a fire that cannot even be described, only experienced.
Some speak about God creating the world as an act of kindness. The truth is, that when the Rabbis talk about accepting the Yoke of Heaven (Torah), they don't really talk about why one should accept this Yoke. If God created the world as an act of kindness, why does it matter whether finite man accepts or doesn't accept the Torah? Does He care?
Maybe the answer is that the fire-that-burns-but-doesn't-
Pirkei Avot teaches a man not to get arrogant if he knows a lot of Torah, because it is for this he was born. Prayer, Torah, Good Deeds, allow us to touch God, and for this we are indebted to His kindness.