Chemical reactions occur because different types of molecules interact with each other to create new substances. Many of these interactions happen naturally and are not man-made. When one molecule interacts with another one, a chemical reaction occurs due to the breaking of old bonds and the forming of new ones.
The chemicals have to be in contact with each other in order for any type of chemical reaction to take place. One of the most common examples of a chemical reaction is the rusting of a fence. The combining of the metal and the oxygen causes bonds to break and new ones to be created, which results in the creation of iron oxide or rust.