And the fire upon the Altar shall be kept burning in it... and the priest shall burn wood on it every morning (Leviticus 6:5)
Although a fire descended from heaven upon the Altar, it is a mitzvah to add to it a humanly produced fire.
(Talmud, Eruvin 63a)
This is a rule that applies to all areas of life: the gifts of life are bestowed upon us from Above, yet it is G-d's desire that we add to them the product of our own initiative.
(The Chassidic Masters)