Formation of a solution is a physico-chemical process. When two substances mix to form a solution, heat is either absorbed (endothermic process) or released (exothermic process). This depends on various interactions taking place between the solvent and the solute at the molecular level.
Glucose exists in the crystalline form. When dissolved in water, the crystal structure is broken. To break the bonds in the crystal energy is required. This is obtained from the water itself and so its temperature is reduced. Chemists call this an endothermic process. But considering a similar reaction, the dissolution of salt (sodium chloride) in water.
Though this is also an endothermic process the heat transfer involved is very less. (Moreover there are other interactions of the sodium and chloride ions with water, which are exothermic in nature).
Strong exothermic effects are observed in certain cases where the substances interact strongly with water molecules. For example, dissolution of washing soda (sodium carbonate) or sodium hydroxide.