Revisiting this question--I'm pretty sure the reason why it's wrong is because you need auxiliary verbs to ask questions in English (in this case, "to do"). For example, "Do you want to build a snowman?," "What does he like?", "Why does he eat so much spinach?"--you wouldn't say "Want you build a snowman?" or "What he like?" or "Why he eat so much spinach?"
Although you can say "You want to build a snowman?", it's casual. I don't know if it's wrong, but I would never phrase a question like this in writing.