…because there is an increase plasma osmolality or a decrease in plasma volume.
Taking up extra salt decreases blood volume as more water is pulled out from the surrounding fluid (because salt is in the extracellular fluid and is increasing osmolality), and arterial pressure increases. When arterial pressure increases, more fluid with more Na+ passes through the macula densa. Macula densa senses the high amount of salt and releases a signal to decrease renin and so decrease angiotensin. Angiotensin is responsible for the release of aldosterone, which is meant to be the salt saving hormone. Therefore, less salt is saved, and more of it leaves the body. Water follows the salt along osmotically, so you become thirsty.
If you increase your protein intake by a lot, the increased amount of nitrogen you consume could mess up the macula densa system.
Another way you become thirsty is by lowering water intake (dehydration). Since there is increased extracellular osmolality, the brain recognises this and sends over ADH (antidiuretic hormone), which binds to the blood side of the collecting duct (in the nephrons of the kidney). This increases water reabsorbtion (water —> interstitial space —> vasa recta —> blood stream), and less water is excreted.