Chemical reactions occurs between different chemicals. They usually occur because different substances have different properties. Certain substances have higher affinities, whereas others have unstable valence electrons. This results in different reactivity of the compounds. When these substances interact, the substance with the higher affinity takes over and cause the chemical reaction.
In addition, there is a major role of bond energies in the process as well. Where there is a lower bond energy involved, the activation energy is lower and the reaction can proceed. Activation energy is the energy required to initiate a chemical reaction. Hence, when the activation energy is provided and the bond energies are low, the reaction proceeds simultaneously.