The question "Why do characters want things?" can often be rephrased as "So why exactly is there a plot?"
I've been working on a certain short story for a few days. I had most of the characters sketched out and basically knew their personalities. I'd detailed the history of the city where the story takes place, and given sparser (but enough for my purposes) history to the rest of the world. I'd figured out how the protagonist would interact with the other characters, and written some dialogue between them that looks pretty believable. I knew the primary conflict and a secondary conflict. I had gone so far as to write the climax out entirely, even though I consider that to be almost cheating.
Then I realized why I was cheating. I had no way to plot the story up to the climax, because the protagonist had no reason to get involved with any of this. There was no motivation, and everything that I could think of cram in there was . . . well . . . obviously something I just tried to cram in there.
The story isn't a complete wash, I just need to start from the characters and build my way up to a plot. Going the other direction is easier sometimes, but it results in characters that tend to be a little too vacuum-sealed to the plot. It's easy to fall into the trap, "if it's faster, it must be a good idea!" It's important to remember that characters should generate the plot, not you. (Of course, well-written characters will usually generate something pretty close to the plot you want.)