At one point in an interview, David Fincher talks about how he wanted "a reason to make a movie, not an excuse to make a movie."
Nailed it! I thought. If there was ever a more succinct distillation of why too many of the wrong kinds of movies get made — or why too many of the wrong kinds of things, period, get made — this is it. A property or an actor is "available", and so a project is thrown together to exploit their availability. The larger question of whether anyone is out there pounding pavement, hungry to see such a thing happen (aside from the fact that they'll get paid much long sugar to do it) never comes up.
Movies are the go-to worst example to see this mechanic in action. They cost so much, and require so many people pulling in one direction together, that the fact that anything makes it to a screen of any size is a flippin' miracle. Something borne out of a single person's burning need is orders of magnitude harder to realize. Every now and then it happens, but typically only because the person in question has already paid some dues or has blackmail photos involving sheep and yards of rubber tubing.
By contrast, a book's a great place to make such things happen. It costs next to nothing to produce; it generally requires only one person to make the effort; and the channel from that person to the finished product is short and almost entirely unbroken. It's far easier to find a reason to do something like that, rather than an excuse (e.g., YA dystopias are flying off the shelves right now, so let's ride that bandwagon while it's still over 50 MPH).
To that end, the big distinction between a reason and an excuse seems to be intrinsic vs. extrinsic motivations. I've talked before about how my main motive for most of my writing projects amounts to, "Why isn't there a story about this?", and so I spend a year and change filling this gap in the world that only I can see. The motive comes from inside. Sure, I want other people to read it and be curious about it, but that's a given; the prime mover is "why doesn't this exist?" The flipside of that is when you look around and say "This kind of book sells well, I could do that", and then you end up chasing the rainbows of public taste.
Many creative types don't conduct a whole lot of investigation into personal motives. The impulse alone is enough for them, and for the most part they are entirely justified in such thinking. But I encourage it — not as a way to be down on themselves, but to understand all the more clearly why they are drawn to it. If at the end of the day they'd really rather be somewhere else, doing something else, they should face up to that. If, at bottom of it all, they have an excuse and not a reason, that needs to come out.