This is one of those fundamental rules of writing, the ones nobody questions, like 'write what you know' and I'm going to question it, because to me it's a ridiculous question. My characters don't know what they want, in the grand serious categories, any more than I would if you asked me.
One of the hardest things I've found to learn as a writer is that things that happen in real life don't translate into fiction. In real life people's motives and thoughts are cagey and secret. Circumspect. And you can't get away with that in writing--supposedly--because everything has to be spelled out on the page. But there's a balance there, between telling your audience everything they need to know, and telling them everything they need to know.
The Star Wars holiday just passed, so I'll use an analogy from that. We need to know Luke's father was a Jedi, even an important one, from early on. The intention is for that to become a central part of his character, so we need it. But there's a...finesse, to giving us the information. Could Obi Wan have sat Luke down and hashed the whole thing out, beginning to end? Absolutely. And I bet, given it was Alec Guinness and space opera we'd even have listened.
But we wouldn't have cared. All that dramatic tension comes from Luke's quest to be more like his father, to live up to that.
There's no hard and fast rule for choosing what details to give and where to give them, no guideline that'll tell you where to but your grand realization (I'm a fan of as late as possible as long as it doesn't require twisting into gigantic Deus et Machina circles).
I spend a lot of my writing life reminding myself I don't like it when complete strangers over-share their lives with me. Other readers probably don't like it either.