Medicines, particularly diuretics, and disease conditions often seen in older people make fluid consumption more than a casual affair. Neglect of symptoms and poor compliance with medications can be very dangerous. Severe constipation can also complicate dehydration. Our hunter-gatherer ancestors only had water to drink. Nowadays, however, it seems almost no able-bodied person drinks plain water.
Fluid comes in various colors and temperatures, with varying bubbles and other complications.
But plain water still claims many advantages. It has no calories, or salt, and it does its job of lubricating us just fine. Then there are the external factors. Heat causes increased fluid loss through perspiration, which can, on occasion, be extreme and result in substantial dehydration. Infection, when accompanied by fever, clearly imposes an increased fluid demand, and colds in particular benefit from more fluid intake, as fluids help keep the mucus from getting too sticky and difficult to eliminate.