If you ever want to really understand how the doshas interact in the body, build a campfire and watch it. The wood won't light unless it is dry enough (Kapha must be low and Vata must be high). Unless there is enough air (Vata) available to flow through the wood, it will not burn. If the wood is dry and air flow is good, the spark of fire (Pitta) will quickly become a strong and roaring flame. The fire will catch even more quickly if you add a wax fire starter (a fatty, Kapha substance).
In the same way, Pitta in the body will move quickly to places it doesn't belong if Vata is unbalanced. Vata dries out the Kapha moisture in the body and makes it more vulnerable to Pitta inflammation. Vata also moves quickly when unbalanced, and the situation inside the body is then like the wind blowing on a fire. An example of one such condition is irritable bowel syndrome, where stress in the nervous system creates inflammation in the digestive tract.
The flames of Pitta are made stronger by the motion of Vata, and fueled by Vata and Kapha. Kapha can create blockages in the body, which prevent Pitta from moving where it should, and cause extra flow in other places. One example of this is coronary artery disease, where arterial blockages prevent or restrict blood flow.
Build a campfire and watch the elements interact. Or just build a fire in your fireplace, backyard grill, or whatever you have available. The best way to understand the doshas is to observe them in nature. As your Pitta fire burns, you will notice how the heat completely evaporates all of the Kapha moisture in the wood. The wood dries out and cracks, eventually losing form completely and turning into dusty ash. This process illustrates how Pitta can increase Vata: fire burning too hot and uncontained causes destruction. Vata has a catabolic (destructive) quality, which is enhanced when Pitta is acting upon it in an excessive way.
In our bodies, Pitta in excess creates Vata imbalances. Examples of such conditions are chronic and adrenal fatigue, also known as "burnout". We push ourselves to do too much, never taking time for stillness and play, until one day we discover that we can no longer push. Our adrenal system is dry and tired, and we have no energy. Our Pitta has burned out all our Kapha. Kapha gives us our vitality (known as "ojas" in Ayurveda). This burnout creates a Vata state of nervousness, worry, fear, poor sleep, and low endurance.
When it is finally time to put out your campfire, you probably will smother it by throwing dirt (Kapha-earth) on it, and then by pouring water (also Kapha). Now you see how the Kapha can either contain the Pitta, or increase it, depending on the conditions.
To make a successful campfire, all the elements must be present in the right balance. The wood must be dry enough, air flow must be optimal, and a good fatty fuel will enhance the ability of the fire to catch. The fire must also be contained by an earth-type substance such as rock, metal, or earth, in order to keep it from spreading.
This campfire situation also exists inside our bodies. We need enough of each dosha to perform the functions of life, but a dosha moving in a wrong place will cause problems. Health in Ayurveda is about learning to maintain a balanced state of the doshas, not only in the physical body, but in the emotional and spiritual bodies as well. Learning about how to create this balance is a process of learning by experience, observing the world around us, and learning to trust our intuitive knowledge. We are all part of nature so we all have this knowledge within us. Ayurveda teaches us how to awaken this inner healing knowledge and claim our birthright of health and wholeness.