Is the reaction between methane and oxygen endothermic or exothermic? Explain why we burn fossil fuels, such as natural gas.

The thing that governs whether a reaction is exothermic is the energy given out / used up to break / form the bonds in the reaction.

When two substances react, the bonds in those substances first break up, releasing energy, before re-forming in a different way, taking in energy. The nature of the bonds that are broken up and reformed determines whether more energy is given out (exothermic) or taken in (endothermic)