The first thing is that if prerequisites are WANT and EMOTION, then the "want" that drives your story had better matter a lot, otherwise nobody, least of all your protagonist, is going to maintain interest. So the want might well loom up into the nature of obsession, a la Ahab and the whale.
But "WANT" isn't the same as "NEED" and that gives us another clue as to the nature of story. And the satisfying nature of the more complex version where what a character WANTS is not the same as WHAT IS GOOD FOR HIM or, perhaps, no hero can win until he has shown he deserves it. Or she, naturally.
But I digress.
Let's consider Jack and the Beanstalk and Cinderella.
They begin in poverty (and in the universal fairytale condition of either orphaned or down to one impoverished parent).
In the case of Jack the Beanstalk want and need arrive hand in hand, and Jack makes the (on the face of it) disastrous decision to trust some glib traveller and swap the last thing he and his mum have for some magic beans. But ultimately his decision is exonerated as the beans grow into the beanstalk and he climbs it, steals the golden goose and slays the giant. All it lacks is a Princess to marry and the throne of the Kingdom, but you get the gist.
But right at the start of the story we have two drivers...the change, impoverishment, and the action, going off to market to sell the cow and opting for magic beans. He appears to have been duped, but the beans really are magic.
Digressing, you could argue that the story is an allegory for those who raise herds to move over to raising crops. In a way all seeds are magic, even if they don't grow over night. But back to the point.
So having made the decision to trust the person, or thing, that sold him the beans, Jack is equally bold in his actions thereafter. He climbs the beanstalk. He makes an ally of the Giant's missis (that's a bit odd, ain't it?), but I suppose that's only one version of the story. Wasn't there also one where she is short-sighted? Anyway, in all versions he steals the goose that lays golden eggs and hares it, pursued by Giant, and chops down the beanstalk quick enough so the giant falls to his death.
Now the giant is obviously both cannibalistic and doesn't like Englishmen, but has he done anything else in the story to deserve this fate? In some ways he plays exactly the same role in the story as the Dragon who guards the hoard of gold and is slain by the Knight or whoever. It is just an obstacle to be overcome, whether by riddle, by magic sword, slingshot, subterfuge or maybe even tamed, it's just there to be gotten past to get the gold.
And we come to the want and the need. He needs to eat, but he gets untold riches. The WANT appears the moment he sets eyes on the Golden Goose. Another Mcguffin. He now wants the magic money machine and must slay or otherwise cheat its owner to get it.
Now I'm not sure how the Giant got to own the Golden Goose but Jack doesn't have any claim to it, except that he stole it. Now historically speaking the guy in the castle on the hill owned everything and everybody and going to steal his money would have got you killed. But you could always have married the Squire's daughter. But that might be a digression too.
The point is that Jack started with a need and then it became a want and then he gets everything. And the only things remotely deserving that he does to get them are
- he trusts the magical tradesman or woman of fairy with the magic beans (faith or gullibility?)
- he is bold
Now Cinderella begins with a need...she's not exactly hungry (although Jack doesn't seem too hungry in his story either, most of the time), but she is downtrodden and badly treated. There's something about the spirit of her mum in a tree there somewhere, turned into a fairy godmother in other versions. But she wants to go to the Ball (not really a need) but she needs to be rescued.
She has to earn Prince Charming and as far as I can see she earns his love by:
- Being beautiful when decked out by the fairy godmother
- Not being cruel
- the shoe fits her; and
- possibly by leaving the party at the stroke of midnight as ordered
Now that last one is a bit contentious, but if you think about it, she leaves at midnight as ordered and by doing so she appears to have lost the Prince, when staying might have seemed the best decision. You could argue what the story is really getting at is she withheld her virginity from him, and thus earned his later proposal as a properly modest bride. But more likely, we have the more storylike motivation that he is fascinated by the one he can't have (absence makes his heart grow fonder). By leaving she appears to herself to have lost her heart's desire, but actually she becomes his WANT (obsession?), and he starts the shoe trying on search.
Okay these are simple stories but the wants and needs are already getting a wee bit complex (like fight and flight it's only when you look closer that you realise how complex motivations can be).
The thing about stories is that very often a WANT is as likely to destroy a character as save him or her, and very often a character needs to give up what they want to get it. This is especially true in love stories. The character has to see that the thing they wanted is an illusion, or delusion, and it won't bring them happiness. Only when the scales fall from the eyes and the truth is clear can happiness be found ...and they may or may not be what the character wanted in the first place.