Like many other motivations, thirst has several influencing components.  Obviously, there is a large physiological component; not so obviously, however, are the social and cultural components.  In addition, there are environmental cues.  First the physiological component will be discussed.
     When we’re hungry, our stomachs growl.  When we’re thirsty, our throats feel dry and scratchy.  What causes this?  There are two monitors which control our physiological thirst motivation.  The first is one that monitors the amount of fluids inside cells.  When the level of salt in your blood is high, it absorbs water out of your cells, which sets off the first monitor.  The second monitor keeps track of the amount of water outside the cells.  When the extracellular fluid level drops, less blood goes through the kidneys.  The kidneys then release a substance into the blood which is carried to the brain and sets off a need for thirst.
     Environmental cues generally have to do with the season.  In winter, for instance, one is much more likely to drink hot chocolate, while in the summer one is much more likely to drink lemonade.  As for the social and cultural components, our drinking is often based on these.  For example, in England, tea is a very popular drink, whereas in the United States, coffee is more popular.  On formal social occasions, one is more likely to have martinis and wine, while on more casual social get-togethers, one is more likely to drink beer or sodas.
     As we can see, there are many factors which influence our motivation for thirst, and all these components interact with each other as well.