Motivation, please?

A very talented friend of mine recently completed a side project. Not for a deadline or a paycheck, but just for personal improvement, shits and giggles, and because she wanted to. I beam with pride. That shit is hard to do.
For some reason, the vast majority of us humans are hard-wired to need to be forced to do anything not strictly meeting the definition of leisure. Which is pretty dumb, when we all admit we would be happier/healthier/more productive if we just did the things on our To-Do list in a timely fashion. And yet, our personal projects sit un-completed, while our netflix que swiftly empties.
Philosophers, and more specifically economists try to describe and predict human behavior with a few simplifying premises in place. One of these premises is that we are rational, which is clearly flawed. There are new theories being tossed around, trying to explain why a rational person would do irrational things, concepts like "bounded rationality" and "time inconsistency" get tossed around to explain why we basically act like teenagers when it comes time sit down and work one something without an external force pressing us.  The basic idea is that we think differently about the present than we do about the future, in terms of wants, desires, money, and expectations.  Which makes sense to anyone who ever spoiled their dinner with a sugary sweet treat because someone brought doughnuts to work.
Another theory is that of supernormal stimuli, which basically says that our recently evolved brains haven't caught up with all of the amazing, yummy, shiny things in the world, and so rather than consume them in any reasonable fashion approaching moderation, we go for it with the all-you-can-eat buffet of sugar, fat, fun, and leisure. Think eating State Fair food while sitting in a hot-tub recliner watching every season of your favorite show in a row. With beer. Who wouldn't want to do that?!
I suppose the boring answer is, an evolved adult human being with shit to do, like, walk the dog, read that book I bought 2 years ago and/or wash myself. Booooring.
This is of particular concern to me because I have a pretty large project staring me in the face, and after a short burst of enthusiasm, have completely halted production. I have, however, used up all my lives on whatever online video game was handy so, there's that. But actual work on a project that I care about and will contribute to my future in a real way? Nah, I'm going to need a deadline or, better yet, a troll with a giant mace standing behind me to get that done. And maybe take away my internet connection.
Luckily, human beings are highly adaptable creatures, meaning we can always change.  Seek out new civilizations and boldly go back to what we started, and finish it.  Good habits, though much less fun, are just as plausible as bad habits.