Energy is absorbed in order to break bonds. Energy is released when bonds are formed.
Enthalpy is related to bond formation and breaking by:
ΔH=∑ΔHbonds broken−∑ΔHbonds formed
Since ΔH is negative,
ΔH<0
∑ΔHbonds broken−∑ΔHbonds formed<0
∑ΔHbonds broken<∑ΔHbonds formed