The thing that usually sidelines a build for me is bumping up against a skill block. Since getting "serious" about the hobby about six years ago, the only thing I've been really successful building is the stash. That said, I think I've overcome enough skill deficits to finally complete some shelf queens and participate more actively in the Forums.
What Jay Jay said really resonated with me...it's not so much about the destination as it is about the journey. I really enjoy the learning process; about the subject, the kit, and the skills. My display space is very limited, so most completions will be given away or stored in totes. Fermis, if you're reading this, I envy you your Model Kingdom!
There's a tote underneath my bench that has some almost complete models to grab when time & motivation coincide.
Happy modeling!
Mike