The Department of Physics at the University of Illinois states that sugar dissolves faster in hot water than in cold water because hot water gives sugar crystals energy. The hotter the water, the higher the energy, and the faster sugar crystals dissolve.
Sugar crystals dissolve faster in hot water because hot water breaks down the tight, low-energy molecules of sugar. Sugar crystal molecules are held together by rigid bonds, meaning their energy is low. In order for those bonds to break up, the molecule needs more energy. Hot water, on the other hand, has a lot of energy as its molecules bounce around its container. When sugar is added to hot water, the molecules of the sugar are relaxed and broken down within the water.
There is a point where the sugar-to-water ratio is too high and the water is over-saturated by the sugar. In such a case, the sugar that is not broken down will remain in crystal form and fall to the bottom of the container, even if the water's temperature is quite high. If a container of hot water and sugar is left out to cool down, some sugar molecules will gather back together and reform sugar crystals.