Basically when two atoms react with one another they need to come in contact with one another. So in other words they have collide with one another, when this happens they release energy namely in the form of heat (some reactions release energy in a different form, light for example; chemical luminescence) this is for exothermic reactions.
For endothermic reactions there is more energy needed for the atoms to react (collide with one another) then there is released namely energy in the for of heat is needed.
For the reaction to take place thermic energy is used, meaning that the surrounding temperature drops.
(Exothermic reactions produce energy in the form of heat)
(Endothermic reactions use energy in the form of heat (from the surroundings) meaning that the surrounding temperature drops)
I hope that this information will help understand why Albert.