This is something that is well-known in writing circles. Almost all fiction can be summarised as 'Obstacle arises or something terrible happens. Protagonist tries to deal with it.' He or she may, or may not succeed, but without those problems, there really isn't a story. 'People have a nice time,' simply doesn't work.
But what I haven't seen discussed before is what effect this constant need to make characters suffer has on the writer. As writers, we either want to, or have to, write. (For many writers it seems to be the latter. We don't seem to have much choice in the matter.) And to write fiction we have to put our characters in difficult positions and make them suffer. So does that make us virtual sadists, getting a kick out of the suffering of our invented personas?
To date, I have published two novels. The first, a young adult SF novel, Xenostorm Rising, has a protagonist whose parents go into hiding, leaving him to fend for himself. YA authors frequently have to find ways to rip teenagers from the safety of their families, to give them the kind of independence that the Famous Five achieved simply by having a scientist uncle who didn't give a damn. Without that, the YA protagonists can't really do their stuff - but it does put the author in an unenviable position of having to either kill off or remove parents (or make them truly horrible), always a delicate task.
In the second of my novels, the murder mystery A Lonely Height, my central character, Capel, gets off pretty lightly (don't worry, he will suffer significantly more angst in the second book in the series), but of course it's inherent in a murder mystery that a fair number of the characters will be having a horrible time, and it's rare that the detective gets away with Midsomer Murders style domestic bliss for long. (I really wish that simpering wife would do something truly evil.)
Realistically, whenever we venture into fiction, the chances are that one or more characters is going to suffer. So, while you don't have to be a sadist to be a fiction author, it arguably makes the process of writing that bit more enjoyable if you are.