We can use fire to cook our food or warm our bodies or to shape and transform metal, but what's important from the standpoint of astrology is the number of ways that we use terms related to fire in language, to describe and understand ourselves, others, and the world around us.
Fire is the closest thing in our world to the basic, intangible substance that the ancients classified as "warm" or "hot." No one really knows how long ago we, as a species, learned to manage or control it, and there is no doubt as to what happens when fire goes unchecked; it can be one of the most destructive forces in nature.
Fire is aggressive. It will take advantage of anything it can reach and use it as fuel to grow. It is also mobile, needing little more than a bit of wind to enable it to burn hotter. This aggressiveness is well noted in astrological literature, and is seen in the fire signs' personality profiles as the desire to get their way.
This aggressiveness also becomes the attribute or opportunism, the ability to think fast and grasp opportunities. In people, this aspect of fire is the basis of risk-taking behavior; it's likely that the person who made up the expression "no risk, no reward" had an abundance of the fire element in their horoscope.
The survival instinct is a part of this opportunism on the biological level. It's what drives living things to fight for their own existence. In the human personality, it also becomes impulsiveness; the tendency to "jump the gun" or move forward quickly and deal with the consequences later.
You will often find that people speak of the "fire of life" as a measure of how energetic a person is, and almost everyone on earth has an expression that means the same thing as "fiery." It doesn't mean that a person with a predominance of the fire element has any more life in them than the next person; it just means that they express it more easily.
When it comes to the emotional life of people, fire is an almost universal metaphor for passion. When people on both sides are passionate about their point, they are having a "heated argument." If a person gives a speech that impassions a group of people, he's called a "firebrand" and when the audience is really excited about what's being said, some say that the crowd is "fired up." If one's passion turns to anger and is out of control, they're called a "hothead."
But that's only one side of passion; the passions are also the basis of so many expressions we use having to do with love and romance. When a person sees someone that excites their passion, they refer to that person as "hot." So many of these expressions have made it into popular songs, there is probably no one who thinks "Light My Fire" was about someone asking another to get their stove working for them. You won't find people who think the old song "Fever" was about a medical condition either.
When the fire element is controlled, however, this "heat" gets turned down a little and becomes "warmth." Warmth and friendliness are characteristics that people born into the Fire Signs, Aries, Leo and Sagittarius, are known for. On the psychological level, Fire people are said to be creative, innovative and pioneering, because they are always looking for ways to expand their territory.
To learn more about the Fire Element and how it helps define the Zodiac signs, jump to the Fire Signs.