Its really hard to say ... I get so many ideas at once, its hard for me to focus on just one story. As it is currently, I'm juggling about six different ideas and developing two almost full-time, others part. But with some of the ideas, they literally just scream "write me!".
When I get stuck on one story, I always have another to go to until the kink works itself out or another plot bunny drops in my lap. While it does take longer to produce a finished work when doing so many at once, I'm never out of ideas to function with. A break is still a break, whether or not its on just one tale.
In all honesty, though, I think its the characters that make me want to write. You can have the most fantastic plot in the world, but if you have bland characters, no one will remember it. Its the people in the story that drive it.