How do UFOs come about?

Dear users, dear authors,
why do you stop working on some models? How do UFOs—unfinished objects—come about? Why do you lose interest in some patterns, while you complete others in their entirety?
Let me know.

I’m not one of those “mass UFO producers”
—I generally finish my current project first (I want to see how it turns out in the end), though occasionally I’ll squeeze in a little something in between.
However, I’ve had UFOs too—sometimes because the yarn was just no good. For example, I once bought some velvety ribbon yarn to crochet a sweater. But the yarn felt really nasty while I was crocheting, and it got all statically charged. I left that project lying around for ages and eventually cut it all up and used it as stuffing.
Sometimes, though, I don’t really like the pattern after all, or it’s too complicated, and then I lose interest. I only unravel it once I have a new idea for the yarn.

I always have several projects on the go. Sometimes new ideas pop up that I just have to try out. Or I want to finish multiple projects at the same time.

Often, I’m working on a complicated project that requires my full concentration. And then I need something else on the side—something simple that I can knit or crochet even when I’m with other people.

UFOs happen when, for example, I get stuck on a project. Maybe I don’t like it, or the yarn is just no good. Or I’m waiting for a flash of inspiration on how to continue... ;-)

Well, and sometimes I just lose the desire to keep working on a particular project because I find something else much more exciting at the moment.