I don't think I experience this problem too much. When I have an idea that seems important, I tend to revisit periodically, so I will continue writing. If I have an idea which doesn't seem that important, but I'm interested in, I usually drop it. Of course, maybe this is purely a selection effect.
I am a person who used to hate writing and now finds it mildly fun, so perhaps David_Gerard's point applies.
Steve Pavlina explains that the method he'd been taught in school-- a highly structured writing process of organizing what to say before it's written-- tends to produce dull writing, but starting from enthusiasm results in articles which are a pleasure to write and are apt to be more fun and memorable to read.
This looks like PJ Eby territory-- it's about the importance of pleasure as a motivator.