I found also this, which I find very good for me as a teacher, cause somehow has some pedagogy in it, please note the difference on the last paragraph between 'state' (imperfecto) and event (indefinido):
Here is why (I think) you are having this problem. Native English-speakers have trouble distinguishing between preterit and imperfect. So they are taught to recognize "signs" of the preterit and imperfect. One sign is the use of siempre, which can of course signal the imperfect (She always went to church on Sundays). But the students cling to these signs like a shipwrecked sailor clings to a log. If you tell them that they can't rely on this sign, they feel they will drown in an ocean of confusion.
You have to get the students to realize that there are no magic words and the difference between the preterit and imperfect is the point of view of the speaker. Words used in the sentence can give an idea as to the speaker's point of view, but it's the point of view that controls, not the words.
My attempt at explaining, is that this is a narration of events, which typically uses the preterit. I view the quiso as an event, rather than a state of wanting, because it is implied that for the entire time in question he had the desire, which was terminated.