I was actually thinking about this today, after I successfully planned out the beginning of my next fancomic series...which are historically the only series that I can even consistently work on...
I don't always finish fancomics, but they tend to hold my attention for much longer, and they usually run long enough for me to satisfyingly 'wrap' them, even if I don't do everything I originally planned.
Meanwhile, original comics...I have a very bad track record with them. 9_9; I've finished exactly one in the modern day, and I'm currently having to scrape together motivation to just continue with its second season, let alone finish it.
Why is this? Well, here are the qualities of the comic projects I usually finish/wrap:
1) No schedule. I update as much as I want, whenever I want.
2) Fast and loose plot progression. If I really don't feel like drawing a certain part of the story, I simply narrate it or skip it entirely. All the visuals are things I saw clearly in my mind's eye and particularly wanted to put down on paper...basically, the comic is made up exclusively of 'the fun parts'.
3) New artstyle. Coincidentally. ^^ Although coming up with a brand-new artstyle is a lot of fun and probably helps with morale.
4) New formatting. This is something I think I should actually investigate in order to help with all my comic work...certain formats (like smaller pages) help my artist mind more than others.
The first two qualities are the most important, and unfortunately they pertain pretty much exclusively to fanworks: like, of course it's easier to go without a schedule when your work has a built-in audience. And of course it's easier to skip around when your story is based on a pre-existing one, and your characters already have established personalities.
But that kind of freedom is apparently what makes me actually enjoy my work enough to finish it...if only I could apply it to my original works...TT__TT
...I mean, I guess I could. And my art is definitely solid enough for it to still attract some fans (especially if I'm only drawing the best parts of the story)...but that loosey-goosey process, although fun, just feels too 'unprofessional' for my personal masterpieces. ^^; Maybe when I hit 30 and start to feel the grim reaper closing in I'll throw professionalism out the window, but right now I still feel like I should hold myself to a higher standard.