Why does a charged capacitor discharge when a resistor is connected across it?

One must understand the function of the capacitor to answer this question.  A capacitor is a device designed to store electrical charges in an electrical device.  A resistor is a device designed to dissipate some electrical charges while allowing some to pass.  When you add a resistor to a charged capacitor, one that is already storing an electrical charge, you are completing the circuit, allowing the current to flow.  Some of the charge will be dissipated by the resistor, but the bulk of the charge will flow through the connection of the resistor to the capacitor.  As the charge flows through the resistor, the rate of loss of charge is proportional to the voltage.  It is also proportional to the total charge through the whole system.  The resource I have added makes the analogy of a container of water with a hole in the bottom.  The water will flow greatest when the container is at it's height of fullness, then the flow will decrease as the amount of water decreases.