Shakespeare uses the recurring symbol of fire in Romeo and Juliet in several different contexts but always with respect to intense emotions. Fire symbolizes the intense emotions of love and hatred and helps to illustrate the both of these themes. In addition, the symbol of fire serves to illustrate just how unstable intense emotions are and just how much damage they can cause.
One of the first places we see the symbol fire is in Prince Escalus's first speech in the very first scene. Here, he relates the raging emotions the Capulets and Montagues feel for each other to fire, as we see in his lines:
What, ho! you men, you beasts,
That quench the fire of your pernicious rage
With purple fountains issuing from your veins! (I.i.79-81)
The image of fire paints the picture of rage being a red, hot, out of control emotion. More importantly, this speech is all about the damage that the two families' feud is causing, such as causing three whole-city brawls. Therefore, the symbol of fire also shows us the damage that hatred can cause.
Another place where the symbol of fire appears is when Romeo first meets Juliet. He likens Juliet's beauty to a flaming torch, saying, "O, she doth teach the torches to burn bright!" (I.v.46). Likening Juliet's beauty to a flaming torch also likens the intense feeling of passionate love to the burning of a flame. More importantly, a second place in which the symbol of fire is used in relation to love is in Friar Laurence's lines just before he marries the couple. Seeing that Romeo is far too young to truly know what real love is and also seeing that their love is nothing more than infatuation, Friar Laurence warns, "These violent delights have violent ends / And in their triumph die, like fire and powder" (II.vi.9-10). Since powder or sand has the ability to suffocate fire and put it out, Friar Laurence is warning that their passion is likely to die. Interestingly, these lines are also prophetic since the couple actually does die in part as a consequence of their rash, impetuous, emotionally driven decisions. Hence, again, Shakespeare is using the symbol of fire with relation to all intense emotions to show that acting upon intense emotions rather than reason can have dire consequences.