The first step is to conclude: What does Jack want? More than anything else in the world. What he has his heart set on.
And the second step is to consider the reasons why he doesn't want to get it. (Some writers prefer to phrase this: what does he want that he can't have if he gets it. I find "why does he not want it?" more evocative.)
Any sort of contradiction in character, if done well, will develop the character: if Jill is sometimes sweet and kind and sometimes cruel and we are convinced that she is really the same woman, she is deeper than a Jill who is all one or the other. But I find developing the desires the best way to do it.
One way to jog ideas is to look at Maslow's hierarchy of needs: physiological, safety, loving/belonging, esteem and "self-actualization." The last is misnamed, because it is the one least occupied with self, but with whatever the character has dedicated himself to. Still, you go up and down it, itemizing all the ways that a certain aim would fulfill the goals, and then all the ways it would thwart them. And, of course, the same aim can thwart and fulfill the same need. Stealing puts the character's life in danger but allows him to eat. Achieving may win his father's approval but convince his schoolmates that he is doing it to show off. Etc.