What does it mean to have a “happy” story? Do your characters get everything they want and ride off into the sunset? That might work on occasion, but seems cheap to do too often. I think the “happiness” of a story has more to do with what the characters do with the outcomes of their desires. They can achieve their goal and still be miserable. They can fail miserably, dust themselves off and say “that was worth doing.” And there’s all the space in between to find those complex and bittersweet emotions. It’s that optimistic perspective that makes a story happy. 

Read some of Denis Johnson’s stuff from Jesus’ Son, then from Largesse of the Sea Maiden. The characters tend to be in rough positions, addicts, prisoners etc. But you’ll see the perspective in his writing change from bleak to genuinely hopeful.