Originally Posted by
Photolysis
It's not a mistake. It's called the laws of physics, particularly the electromagnetic force which is almost completely responsible for chemical interactions, and statistical thermodynamics. which governs what chemical states are explored in a system.
There's no 'deciding' in chemistry. Atoms do not randomly decide to combine to form molecules. They do so under a certain set of conditions, and providing these conditions are the same, the reaction will always be the same*.
*Not strictly true in a quantum system, but in macroscopic systems this is the case.