The purpose of biological drives is to correct disturbances of homeostasis. Unsatisfied drives are detected by neurons concentrated in the hypothalamus in the brain. These neurons then produce an integrated response to bring the drive back to its optimal value. For instance, when you are dehydrated, freezing, or exhausted, the appropriate biological responses are activated automatically (e.g., body fat reserves are mobilized, urine production is inhibited, you shiver, blood is shunted away from the body surface, etc.). Your body automatically responds to these survival drives. But it is much faster and more efficient to correct these disturbances by eating, drinking water, or actively seeking or generating warmth by moving. Therefore, the body is motivated to engage in whatever behavior is necessary to fulfill that drive. One way that the body elicits this behavioral motivation is by increasing physiological arousal.
The Reward System
Motivation is strongly linked to biological factors that control reward sensitivity and goal driven behavior. Reward sensitivity is located in the mesolimbic dopamine system. Research shows that individual differences in neurological activity in this area can influence motivation for certain goal driven behaviors that will elicit a reward or satisfy a craving. In this way, the reward system spurs physiological arousal which motivates the individual to engage in whatever behavior is necessary to satisfy or relieve arousal. For example, substance use is associated with overactivity in the dopamine system in a way that some individuals are more motivated to engage in substance using behaviors.
To show how the reward system works, in the early 1950s, Peter Milner and James Olds conducted an experiment in which a rat had an electrode implanted in its brain so the brain could be locally stimulated at any time. The rat was put in a box, which contained a lever for food and water, and a lever that would deliver a brief stimulus to the brain when stepped on. At the beginning the rat wandered about the box and stepped on the levers by accident, but before long it was pressing the lever for the brief stimulus repeatedly. This behavior is called electrical self-stimulation. Sometimes, rats would become so involved in pressing the lever that they would forget about food and water, stopping only after collapsing from exhaustion. Electrical self-stimulation apparently provided a reward that reinforced the habit to press the lever. This study provided evidence that animals (and humans) are motivated to perform behaviors that stimulate dopamine release in the the reward center of the brain.
Traits like impulsivity and sensation-seeking predispose people to engage in certain behaviors. These traits generally develop at a very young age (if not prenatally) as part of the individual's temperament. Temperament is defined as an individual's basic way of interacting, and includes aspects like frustration tolerance (i.e., the ability to withstand frustrating situations without getting upset), delay of gratification, inhibition vs. impulsivity, etc. All of these factors play into how motivated one will be to engage in certain behaviors. Fulfilling the impulse brings about a physiological reward similar to the rat pressing the button. Someone who is very impulsive and uninhibited might be very motivated to go buy a car on a moment's notice, as compared to someone who is very inhibited and has difficulty initiating actions.
Likewise, some individuals are more sensation-seeking in that they are drawn to arousing or physiologically stimulating activities. These individuals are more likely to engage in risky behaviors like driving fast, riding roller coasters and other activities that get their adrenaline pumping. Such individuals will have high motivation for activities that they find physiologically arousing.