FIRE seems to exist in a realm somewhere between our material world and the spirit world. On the one hand, it has no substance and behaves in a very spirit-like way, constantly changing its shape, disappearing and just as quickly appearing again. On the other hand, the altering effects of fire on our physical world are very real.
Similarly, fire has a dual nature - it's both good and bad. It is a giver of light and warmth, but also of pain and destruction. It's an element that has the capacity to be everything from beneficial and comforting to terrifying and catastrophic.
Throughout history, symbols have been created to represent the element of fire, and like most symbols they've come to take on other meanings and convey other concepts as well - emotions, qualities, beliefs, etc that people naturally associate with this complex and potent force: Passion, intensity, courage, strength, cleansing, renewal, motivation, and action.
The awe and respect inspired by fire is such that Wiccans often place lighted candles on the south side of their altars to help them channel energy and intensity into their rituals or spells.