For a seed to germinate, the right temperature, humidity and space must all be in place. These are the internal causes. The arising of desire also has three causes.
1. Everyone has the seed of desire, except bodhisattvas of the first bhumi and arhats who have destroyed the seed. For ordinary people, the seed of desire is always stored in the mind even when there is no thought of wanting anything or in deep sleep, just not that obvious. This is the primary cause.
2. External cause. For example, upon seeing your neighbour with a nice car, a thought comes up: I must buy a car better than that one. The thought wouldn’t have come up if you did not see the neighbour’s car in the first place. So, seeing the neighbour’s car is the external cause for this desire to arise.
Psychologists have reported that if we watch an hour of television every day, our weekly expense will go up by US$4. This is because the advertisements on television can induce us to buy things we don’t need. Also, shopping with a credit card often results in spending more than what we would with cash. As a credit card is not real cash, one tends to pretend that no money has actually been paid out. It is, however, a different matter when we feel our wallet get lighter and lighter. This is all just psychological.
Once there is an external cause and condition, all kinds of defilement may arise.
~ Depicted from "THE HANDBOOK FOR LIFE"S JOURNEY : On The Three Poisons - How to Handle Desire"