When you add more sugar to jello, it becomes sweeter because the additional sugar increases the concentration of sugar in the solution. Sugar interacts with your taste buds, and as the concentration increases, your taste receptors are stimulated more intensely, which makes you perceive a sweeter flavor.
Furthermore, sugar can also affect the texture and other properties of jello. Excessive sugar can alter the gelatin's ability to set properly, potentially leading to a softer or more liquid consistency if too much is added. However, within reasonable limits, adding sugar enhances the sweetness without significantly compromising the overall structure of the jello.