Why does an enzyme only acts on a specific substrate?

Enzymes have specific sites which are known as the active sites. The shape of the active sites is specific for specific kinds of substrates. This allows every enzyme to be specific for its action. Only the specific substrate will be able to fit into the active site and hence, activate the enzyme. The presence of active sites makes the activity of each enzyme specific and hence, every enzyme is able to catalyze a specific kind of reaction.