Damned if I can remember who or where, but I did read somewhere that a good novel-length story should have 3 crises and an ending that resolves them. The crises don't need to be overwhelmingly dark or gritty, but that seemed to have been seen as the key. With that in mind, I thought about most of the books I've read and found memorable, and there seems to be something to that theory. 
In a short work, you can have one issue, and as long as you resolve it for the reader, it's all good. But novels need something more, and sometimes the viewpoint that life isn't always pretty, or orderly, or even very fair works for those crises. I think that's why I enjoy doing short pieces here, because it's actually much more of a challenge to create short fiction that holds a reader's attention while letting them enjoy the fapping bits, too.