We can explain this phenomenon if we consider what is called chemical kinetics. A process will have an increased rate if the concentration of the reactive species is increased. Consider the water and sugar solution. In effect, the sugar molecules dissolved in the water get in the way of flow of water from the solution. Second, a pressure develops on the sugar side which forces water faster from the solution side into the pure water.
Osmosis is quite dynamic with pressures and concentrations changing minute to minute.
This relationship holds for any pair of solutions. Regardless of the nature of the dissolved particles, water will tend to flow from the more dilute solution into the more concentrated, seeking to equalize the concentrations across the semipermeable membrane. This simple relationship drove the inventors of Gatorade in their definition of the hydrating beverages. We can understand that salt tablets would be about the worst treatment for dehydration. The body would struggle to supply water from its already depleted supply to the small bowel to dissolve the salt tablets.