The funny thing here is that these countries are usually in the news (bad news, in particular). Mexico's capital city is overpopulated and heavily poluted. Venezuela has undergone several political turmoils. And remember the violence that has gripped El Salvador for decades?
The answer could actually be quite simple: happiness works best amidst challenges. As one Nigerian says, "People smile at you because that is the way we deal with the awful stress in our poverty." Another Nigerian also said they are "happy [because] we don't ask for much. If God gives us food we easily become happy. We are not greedy." One jounalist added, "If you look at our problems it is unimaginable to say we are happy. But then Nigerians appear to have developed a very thick skin."