Why is the reaction between Ammonia and Oxygen Exothermic?

Exothermic reaction:

• A reaction where the heat is evolving out(released) of the system is called an exothermic reaction.
• The reaction between ammonia and oxygen is an exothermic reaction because when ammonia $\left({\mathrm{NH}}_3\right)$ reacts with oxygen$\left({\mathrm{O}}_2\right)$, a large amount of energy is released into the atmosphere, and nitrogen and water vapors are formed as products.
• This reaction is represented in the equation as:

$\underset{\left(\mathrm{Ammonia}\right)}{\underbrace{4{\mathrm{NH}}_3\left(\mathrm{g}\right)}}+\underset{\left(\mathrm{Oxygen}\right)}{\underbrace{3{\mathrm{O}}_2\left(\mathrm{g}\right)}}\stackrel{}{\to }\underset{\left(\mathrm{Nitrogen}\right)}{\underbrace{2{\mathrm{N}}_2\left(\mathrm{g}\right)}}+\underset{}{\underset{\mathrm{Water}\mathrm{vapors}}{\underbrace{6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)}}+\mathrm{Heat}}$