What is the cause of getting thirsty?

Question

Of course we all know when you walk several miles, you become thirsty and same with doing workout.

What I mean here people who are less active and yet they get thirsty.

Answer 1

Thirst has nothing to do with activity and has to do with the level of hydration in your body. 

You lose water FASTER when doing the activities you mention, but those are not the only things that cause your body to 'use up' water. 

Answer 2

Maybe if they feel thirsty they only drink small amount of water. Frequent thirst signify health problems too so better consult a doctor.

Answer 3

I would say someone gets thirsty at times when they are fasting and cannot drink. People may get thirsty also if their mouth gets dry and need to drink something.