When did Buddhism become the religion of happiness?
For example, in addition to the happiness books by Howard Cutler and the Dalai Lama, Jack Kornfield in his Roots of Buddhist Psychology audiobook equating happiness and liberation.
There seems to have been a curious shift away from the Stoic Buddhism of Schopenhauer which dominated the nineteenth century and the first half of the twentieth. What I recall from the representations of Buddhism from midcentury on to around the 80s was an attempt to refine that view—the more nuanced (or at least that is how I recall it being presented, and certainly how I presented it to my own students) view was that liberation was from attachment to either positive or negative experiences. Now the emphasis is on how to achieve happiness, a situation which may not inappropriately be described as the pendulum having swung further along its path, but perhaps toward one of the extremes?

Perhaps this is nothing more than the ongoing commodification of Buddhism for consumption within the framing context of the self-help culture. The latter predefines the goals of human existence in terms comfortable to the consuming public’s own culturally determined preconceptions, thus forming a reinforcing feedback. By entering into this culture, Buddhist authors are of necessity led to structuring their presentations in terms of those generally socially accepted goals, such as, happiness.