So I have an article and one person comments that it is excellent. To make sure that they just don't skim it I ask "thank you. Have you read all of it because I'm afraid it's long", and they say "yes of course". What can I ask next to have a more detailed feedback? Or perhaps asking them to share without being perceived as needy?
"I really want to make this better; please could you tell me just one thing I could improve?"
This helps take the pressure off: you're not looking for a thorough dissection, but something that will take much less of the reader's time.
Like you, I find that a simple, "What do you think?" always gets back a polite, "Very nice, thanks." I do have a few readers with some experience as informal copy-editors, who can return a page with useful notes all over it, spotting typos and complex over-running sentences. But for someone without that experience, who hasn't been asked in advance to review, then asking them to choose just a single point is the most effective way I've found to get more feedback.
I agree with the other answer, that asking for a single area of improvement is a great idea, as it takes the pressure off and focuses on your hunger to improve.
In addition, I would suggest that you really need to get feedback from more than one person, as writing and reading is so subjective you need a range of opinions to get a sense of where there are genuine areas that could benefit from rewriting and which things are just someone's personal taste.
Further to that, it's worth getting feedback from people who are experienced in giving feedback, or at least interested in doing so - it sounds like this person you got to read it doesn't have that much interest in giving feedback.
Using a website like scribophile, where users give each other detailed critiques, could help you get good critique from a range of people.