Gibbs Free Energy??

It says in my book 'many exothermic reactions take place spontaneously at room temperature. the enthalpy content of the chemical system decreases during the reaction and excess energy is released to the surroundings - this increases stability' and it basically says the same for endothermic although obviously the opposite i'm just a bit confused about the second part for some reason: why does emitting heat make an exothermic reaction more stable, and why does taking up heat make an endothermic reaction more stable? or am i just being a bit thick :')