It is satisfying to get things done. I'd like to think that it would be great to always work on projects and finish them in order rather than accumulate a big pile and then work to knock it back down. For me, finished projects are one of my biggest motivators. But I also understand and I am perfectly comfortable with the idea that some projects need time to "simmer" before they get done.
Whether you stay interested in a project is usually a matter of whether the initial enthusiasm for that project has lasted throughout its inception, development and finishing. Sometimes you lose interest half way through or get stuck and need time to figure out a problem. Or you need to gain confidence in your skills or need practice or lessons to improve them or would welcome a new technique or tool before you can proceed. Maybe you are a bit burned out after all the flurry of work done to get a quilt to the finish line and in the closing moments, can't even muster the last bit of energy to finish the quilting or even just the binding. Sometimes the excitement of new projects demand to be immediately harnessed, pushing an old project to the background indefinitely. So generally having one, two, five or ten year old projects doesn't bother me if I still love them and know that they are just waiting for the appropriate inspiration to get finished.
On the other hand, sometimes some projects are slated to be abandoned. If your tastes have changed or you've learned or gotten what you wanted from an experience with a project, sometimes you don't want or need to go on with it. If you are lucky, the seeds of the old project morph into a new project and are used to set it on a different path than originally intended. Maybe you can pass the old project along to someone else who would be more interested in finishing it. As a last resort you can throw it away, particularly if the guilt of holding on to something you KNOW you are not going to finish is standing in the way of moving on to newer and more satisfying projects.
So there are many ways to finish things. But I still say the best part of finishing is, well, finishing!!!