What I mean by this is that if you want to do a lot of something well, you're much more likely to succeed if you unconflictedly want to do that thing day after day.
Here's an exercise example, which if you've ever wanted to get in shape (or succeeded in doing so), may make sense to you: a couple of years ago, I barely exercised at all. Then I moved to Florida and was able to take up year-round running, which was grueling at first but convenient enough that I was able to stay with it. Eventually it got so that I would feel good if I ran and feel lousy if I skipped a few days. I started finding time to go running even when it was a little inconvenient or I didn't feel like it, because I didn't want to start feeling crummy.
More recently I moved back to Vermont and started doing Taekwondo, which has completely replaced running for me. Not only is it a phenomenal workout, but it's mentally challenging, it's social, it absorbs my interest so that I don't have time to dwell on how effortful it is, it's for specific amounts of time so that I don't have to ask myself whether or not I should stop yet, and I get to kick the hell out of things. Since my schedule is a little more flexible this summer, I've found myself going even on days when I feel tired, on days when I can barely spend the time, or days when I have other perfectly valid excuses not to. The process is as enjoyable to me as the gradual result of getting progressively more fit.
Let me bring this home to writing: writing regularly--every week at a minimum for most serious writers, every day for many of us--is sometimes hard. It's especially hard when you're not enjoying the work. In my case, at least, and very possibly in yours, how much you're enjoying the writing (not necessarily the editing, outlining, or marketing, but the generation of new prose) has a lot to do with how excited you are to see what happens next--even though very often you know what's going to happen next. For example, in a novel I wrote a couple of years back, there's a chapter in which the Greek Titan Kronos is released from a pocket universe in the middle of a battle, hoppin' mad. I could not wait to write that chapter, and it drove my writing on.
By contrast, I've found myself in some stories writing something because "this has to happen". Well, sometimes things do have to happen in a story, but then, they don't necessarily have to happen in a way that makes me unexcited to see them unfold. And if I'm not excited about going ahead, that's a red flag for me. Fortunately, there are some quick and easy solutions to that problem. I've recently had to remind myself of these and skip back painfully to rewrite a large section of a book because of it. But the pain goes away quickly, because once the problem is solved and you're excited about the book, it's no longer so effortful and laborious, but something you make excuses to do.
Here are some symptoms and solutions for fixing lack of excitement about writing a story--which by the way, can often translate into lack of excitement for readers as well, a far more dangerous situation.
I took a wrong turn: At some point I made a character act against his or her inclinations, or I threw in a plot element that just didn't belong, and it's poisoned the story ever since. In these cases I need to weed that out and rework the story around it.
Nothing's at stake: Donald Maass in Writing the Breakout Novel talks about two kinds of stakes: public and private. Private stakes are why something matters to the character. Public stakes are why it matters to anyone else. We have to care about the character and the character's interests for private stakes to affect us, and we have to care about the world and the problem posed by the public stakes for those to affect us. If at least one of those kinds of stakes (and ideally both) aren't making us worried about what's going to happen, then we're not going to care.
I should skip this for now: This is hard for me to get through my head sometimes, but stories don't have to be written in order. If there's a section I'm not excited about writing, I have the option of putting in "Here's the part where he has the argument with his mother and they discover they both killed someone" and going immediately to the next chapter, where the cop bursts in. Later I can come back and write that chapter.
The danger with that approach is that there's some underlying problem with stakes or character motivation that's preventing you from writing it, and you need to sort out that problem before you proceed with a flawed story. That said, if you really can't find a problem right now, writing the rest of the story may be the quickest route to doing so, or to proving there is none.
I forgot why I was writing the story: If I get inspired to write a particular story (and I'll talk about inspiration elsewhere, but I'll say here that it's not something I believe writers have any business waiting for, but that instead we must find for ourselves) and later let the story degenerate into details of plot and setting and character, I can lose the fire I had to write the thing in the first place.
There are other symptoms that could fall under this heading, but I'll leave it at this for now and follow up with more in future.
Postscript: Some people may just not enjoy writing under any circumstances, and for these writers not enjoying the process might not mean anything at all. Tim Powers, for instance, describes parts of the writing process as being laborious and unpleasant. But then, Powers writes such enjoyably and mind-bendingly intricate plots that it surprises me his head doesn't explode.