Fire is used to make gold and silver pure, but a person’s heart is made pure by the Lord. (Proverbs 17:3 ERV)
Wildfires burn because there was something to ignite - the fuel existed to allow the slow burn to begin to become a much bigger burn. The wildfire burns because something acted as an ignition point - there was something that set that fire in motion. Wildfires aren't very discriminating about what they burn - anything can be fuel to a wildfire! They also aren't very predictable because they are not contained - they have the potential for limitless impact if there is enough fuel for them to continue to spread. This is why fire fighters will create a "break" between the fire and the fuel - to deny the fire access to what would otherwise have been right in its path. There are times in our lives when we leave too much "untended" areas in our lives that eventually produce enough "fuel" to allow the wildfires of life to burn out of control when there is something igniting that fuel.
A stoked fire is usually a little more contained and is built with a specific purpose in mind. There is a specific type of fuel utilized to create this fire and it is well-tended so it burns at a constant temperature. Why? The fire is intended to warm cold places, light a dark corner, create a means by which nourishment can come into our lives, or perhaps even allow the purification of what touches our lives. It can only bring warmth if tended - an untended, but contained fire will eventually burn out if not frequently stoked. It can bring light into darkness when there is enough glow from the fire to illuminate - but that also means there must be a way for the fire to be in our lives without harming us.
God doesn't want wildfires burning out of control in our lives, so he asks us to deal with the "fuel" of the "fallen stuff" in order to make is less likely there will ever come a time when the fire burns without a real purpose. He desires the "contained fire" for it is purposefully ignited and tenderly stoked so as to maintain just the right degree of "burn" for the task at hand. If we need illumination, he will ensure there is enough flame to shed light where darkness exists. If we are hungry, he will cultivate the coals just hot enough to bring nourishment from what is placed upon those coals. If it is purification, the fire might just be stoked a little hotter than we might like, but what will be produced far outweighs the momentary discomfort of those hot flames. Just sayin!