Sure it also a matter of personal preference, to be more inclined to positive ending stories. No doubt there is enough depressing stuff going on in the world, that there is no need to invent some more.
And of course the "unhappy ending" stories are not a new thing - also here the old Greeks were first and called it Tragedy.
At the end of the day its more important what happens during the story, rather than how it ends. What I'm saying is just, that things are shown in a different way today, especially in movies and TV. In the past it used to be all black and white - not the picture, the story line - good vs. bad, and good always wins. And it was like that for decades.
But thats what people wanted to see and pay for, I guess.