# Why are addition reactions of alkenes exothermic?

Well, you are making $\text{carbon-carbon}$ or $\text{carbon-heteroatom}$ bonds, and given this it is inevitably exothermic.
It is generally observed that bond formation is exothermic, and bond-breaking is endothermic. Olefin reactions generally involve the formation of $2 \times C - X$ bonds of some sort. Of course, we must break an $X - X$ bond of some sort, but formation of $2 \times C - X$ bonds is energetically downhill.