Why do enzymes only catalyse specific reactions?

Enzymes are made up of specific amino acid sequences. Bonds form between amino acids in the chain in specific places. Hydrogen bonds form and fold the sequence into a secondary structure. Ionic bonding, disulphide bridges and more hydrogen bonds form to pull the chain into a 3D tertiary structure. This 3D structure has a very specific shape based on this sequence of amino acids and the bonds which form, and this shape is complementary to only one substrate, so will only catalyse the correct reaction.