I put the pro in procrastinate.

"Procrastination is one of the few skills I've mastered."
~ Ron Stoppable [character on Disney's Kim Possible]

Why is it that some tasks cry out for procrastination and others place no obstacles to prompt completion?

The obvious answer is that I want to put off difficult tasks, but often the job is not all that hard. It may be something that will take less than sixty seconds of my ample free time and utilize a total of four calories of energy. It may not be otherwise unpleasant. But I just don't feel in any hurry to do it. Even when it is something that I know needs to be done and even needs to be done soon -- or at least, relatively soon. 

Yet at certain times, I can sit myself down for two to three hours a day, seven days a week until I have cranked out a complete first draft of a novel of some 120,000 to 150,000 words. 

At which point, I will set it aside for six or ten months, remind myself periodically that it needs to be completed, and then at the gentle and persistent prodding of friends, finally will pick it up and finish rewriting, proofreading and editing it and have it on the market in two weeks.

And then I tell myself that I really should make more of an effort to market the thing ...

But what's the hurry?