There are many factors that go into the growing of a great chile pepper: terroir, climate, seeds, proper irrigation and the like. But it seems that the latter attribute is the most important.
A report published yesterday in Science documents the work of a University of Washington professor who “has been traveling to Latin America to study chilies” (how awesome is THAT?). His research found that “plants make only hot chilies when they don't
have to worry about a drought,” an evolutionary process because the thingy that makes peppers hot (capsaicin) is also what keeps a specific type of fungus away at bay. When there's little water to be had, the chile takes its chances and doesn't produce as much capsaicin–hence, a less-spicy chile.
The article points out that some of the hotter chiles exist in areas with heavier precipitation, such as the habanero and jalapeño, both from the Gulf of Mexico region. But what about the piquin, that tiny ball of hell native to the U.S.-Mexico border? Or the chile de árbol, the pride of Mexico's dessicated north? Regardless, make sure to water your chile plant to ensure maximum heat.