That's something that often worries me with writing. A lot of what helps to make a story good is having the desire to want to write that story. When the desire isn't there, then you're not as motivated to put as much effort into it. And there's a lot that can kill a person's motivation - boredom with the story and characters (which is why I'm nervous about writing series and trilogies. There are quite a few stories I quit because I lost interest in them), the story and characters not turning out the way you want, overly complicated plots, the story not being received positively by the readers, and so on.

I try to only write what I have the desire to write, because it's hard to put effort into something you don't enjoy. Most especially if it's something you're only writing because you feel you have to (as much as I love fic exchanges, I hate it when I end up with a story idea I don't particularly care for, yet I can't seem to come up with anything else).