Chemistry studies changes in matter. A chemical reaction is a process in which one set of chemical compounds are transformed into another. Reaction occur when there is an interaction between the compounds in which some initial bonds are broken and some new bonds are formed.
Why does this happen? In simple terms, because the energy holding the new bonds together is higher than the energy that held the initial bonds. This is the definition of a thermodynamically favored process. Favorable thermodynamics is the most fundamental step that leads two compounds to react with each other. Other drastically important factor is reaction kinetics.