I have lots of UFO's, probably more than most. Sometimes it's because I run into a snag of some sort and set the project aside until I figure out the next step. Most of mine were set aside when I was still working and didn't have time to work on it for an extended time, so I put it away, then out of sight is out of mind. The next thing you know I'm all excited about a new project and off I go on that one.
But I find that I still like to have several projects in the works at the same time, because I tire of each stage of the process and want to switch it up. I get tired of piecing, so I switch to doing some hand work on another project. Then I want to do some quilting, so I switch to the project I am in the middle of quilting.