Too many variables to give a simple, unilateral answer. By happy ending, I suppose I mean the culmination of an ending in which the parties sympathised with triumph and live, and any antagonists are defeated.
First, there's the target audience: is it aimed at children, teens, adults? etc. It is to many more forgivable for a Disney film to have a contrived happy ending than an 'adult' (no, not in that sense) novel.
That's a whole point of its own: a happy ending can seem contrived, out of place, a deus ex machina, but need not be. A fitting, 'happy' ending that has been built towards for a significant part of the story can leave the experiencer satisfied, and the story fulfilled.
Similarly, introducing a happy ending at the expense of closure can result in dissatisfaction, e.g. if exploring the fate of an antagonist is neglected in favour of a simple happy ending.
In the context of a less realistically grounded story, seemingly cheap twists in it can seem more tolerable and acceptable, as they may not break its continuity. In a more realistic story, using an unlikely, unrealistic or plain impossible event to force a happy ending may overstep the boundaries of passable endings.
The expectations of one when starting to have a story told can also influence one's sentiment towards its ending; if one is looking for a quick, shallow story to pass some time, a simplistic, happy ending may not raise objection. On the other hand, if one is looking for a deep, lengthy story for mental stimulation, it would probably be disappointing, perhaps even annoying to finish it without the desired complexity, open-endedness, thought-provoking, open to interpretation kind of ending one may have been seeking.
But even within the course of the story unfolding, one's mood, attention, patience and the like can change; one could start reading a more intellectually-oriented, discussible fable hoping to further one's mind, but part way through become bored or disillusioned.
Of course, it doesn't matter how contrived or poor an ending is if its receiver doesn't realise. To use my earlier example, a simple-minded child could watch a Disney film and be delighted with its ending, while more analytical folks may be disgruntled by it.
As I now see ZeaLitY touched upon, one's own general temperament can affect how much one enjoys a happy ending. An archetypal emo may hate (although, if they hated everything anyway, I suppose this is redundant) an arguably mawkish, 'feel-good' love story with an ending consistent with the rest of it on principle, whereas an old-timer may regale in it. Entire genres and types of endings or plot devices may be written off by people without consideration.
So to wrap it up, one's expectations, temperament, perspective, mood and standards can affect if one enjoys a happy ending, as well as its continuity (or lack thereof) and target audience.
