Why does a change in tempurature indicate that a chemical reaction may have occured?

Chemical reactions are ultimately the reconfiguration of reactants to form products that are in a more energetically favourable state. In this sense, the vast majority of chemical reactions have reactants and products that are at different energy levels.

This means that energy either has to enter or leave the chemical compounds. Some of this energy is in the form of heat, which will cause the system to cool down or heat up.

So, detecting a change in temperature is a good way to find out if a chemical reaction has occurred. Of course, chemical reactions can occur with no detectable change in temperature, and a change in temperature does not necessarily confirm a chemical reaction has occurred.