Here's why:

* Specificity: Enzymes are highly specific, meaning they only catalyze reactions involving a particular molecule or a small group of closely related molecules. This specificity is due to the unique shape and chemical properties of the enzyme's active site.

* Lock-and-Key Model: The active site of an enzyme acts like a lock, and the substrate is the key. Only the correctly shaped substrate can fit into the active site and undergo the enzymatic reaction.

* Induced Fit Model: While the lock-and-key model is a good starting point, the induced fit model provides a more accurate description. This model suggests that the enzyme's active site can slightly change shape to better accommodate the substrate once it binds.

Examples:

* Lactase breaks down lactose (a sugar found in milk) but doesn't interact with other sugars like sucrose or glucose.

* Pepsin specifically digests proteins in the stomach, not carbohydrates or fats.

The specificity of enzyme-substrate interactions is crucial for the precise regulation of biochemical processes in living organisms.