The fire triangle is a simple model for understanding the three key requirements for a fire; a fuel source, oxygen and heat (an ignition source). A fire will occur naturally when these three conditions are present. To extinguish a fire, remove one of these elements.

Love is like a friendship caught on fire. In the beginning a flame, very pretty, often hot and fierce, but still only light and flickering. As love grows older, our hearts mature and our love becomes as coals, deep-burning and unquenchable. ~Bruce Lee

Likewise, a relationship needs the same conditions; fuel, air and a spark. Remove any one of these and it cannot survive.