"Σε θέλω" is not very professional to say. So we could say "σε θέλω να είσαι ο γιατρός μου" (I want you to be my doctor), but it is not very appropriate. We would rather say "θέλω να είσαι ο γιατρός μου".
"Σε θέλω" shows a very strong desire. "I want you" is much stronger than "I want".