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Just arm chair speculation, but it seems like any religion that promised happiness or how to be happy wouldn't be wildly successful. Not many people turn to religion when they're happy, and promising happiness to a bunch of sad people isn't going to turn out very well. Telling people that their lives sucks for a reason, we just don't know it yet, and that at least when they die they'll be happier than they could ever imagine is a pretty easy promise to keep to the living. It also gives people hope which in turn can produce happiness or at least a sense of purpose.