In Shangri-La, there are no native discontents, and the accidentally imported ones vent their discontent on a desire to leave. If voluntary exile from a place one has no liking for is the "punishment" for being discontent, it hardly seems a bad one. Yet those who are not happy in the best of all possible worlds are those short-sighted enough not to realize the consequences of leaving it. Both book and movie rely on benevolent paternalism as a form of government. Father knows best.
In Lost Horizon, happiness is a geographic destination. In Stupid, Stupid Rat Tails, it's experience, the endless process of discovering the world. Although storytelling pervades the story, the stories within are a way of reliving the past and entertaining others who haven't the gumption to live as as Big Johnson Bone does. He experiences the world carelessly and widely. Happiness is action and exploration, in making up the places which lie beyond the map, in passing on to the soon-to-be-known.
In contrast, Terry Pratchett's characters find their happiness in the adequacy of daily life. Daily life, chores and duty, are the reprieve granted at the end of an adventure. Sufficiency is the reward for apocalyptic success. Save the world, and you too can go milk the cows, gather the eggs, sweep the floors. It is enough. The wintersmith is a source of disruption and disturbance, an annoyance, even if one with ego-building side-effects.
Strikingly, in these three stories*, only in Wintersmith is happiness, such as it is, necessarily attained at the narrative's conclusion.
* These all happen to be the books and movie I've watched/seen this week. I also read These Old Shades, where happiness is obtained by the mutual negotiation of positive social relationships, whether friendship or marriage, but that ending was mandated by genre to a degree that isn't true of these others. In my limited experience, there's a great deal of creativity in dealing with insurmountable barriers and the way a plot develops in romance, with greater strictures on the ending. Other genre's limitations tend to apply themselves to other aspects of a narrative.