For me, UFOs come from a project that is 1) too hard, 2) too boring, 3) gets pushed out of the way for a higher priority, 4) isn't working the way I want it to.
Right now my major UFO is the very first quilt top I ever made. It was a BOM and for a full size bed. But at the time, I didn't know how to quilt and couldn't imagine spending the money to send it out. Now I know how to quilt, but don't have a full sized bed!!
I also had a 4-month UFO of my shadow trapunto quilt, because it got set aside to do Christmas gifts. Then, rather than pick it up, I started other projects. Oops. It was just really daunting. I'm getting back to it now though.
My bigggest UFO is my list of things I want to do. They aren't started yet.
The biggest thing for me is realizing when a project just isn't going how I want it to. I bag it up and take it to goodwill, or if it is a real disaster (and small) I've been known to trash them.
Why force yourself to makke something you don't like? Time is too precious for me.