Why are combination reactions usually exothermic reactions?Why are decomposition reactions usually endothermic? (though decomposition of orgainic matter is exothermic. why? )

Let me make one point clear first. There is difference between decomposition of a material( like lead nitrate) and decomposition of organic matter( like faeces).

 Decomposition reactions involve a complex compound breaking up into simpler compounds or elements. This process involves breaking of bonds between the constituent materials of the compounds. This breaking requires energy. As a result, decomposition reactions are endothermic.

        I am just too lazy to type the rest, so vice versa for exothermic reactions.

The decomposition of organic matter (like shit) can actually be told of to be redox reactions. So there.