Each enzyme is specific to the reaction it will affect, it has to do with the chemical makeup of the enzyme. Think of it like the key to your apartment or your house; your key won't fit any other door, and their key won't fit the lock on your door. A good example is amylase, which starts the chemical digestion of starch into sugar. Amylase is present in human saliva, causing the starch in potatoes and breads to convert to sugar while it is in the early stages of being introduced to the digestive system. Another thing to consider is what you know about chemistry in general, not everything will react with everything else.