Anybody know how to engage beta readers without sounding like an asshole?

Here’s the problem. I know my book needs work. It’s my first completed novel. What I don’t know is what needs to be modified from the perspective of the reader (ie: What doesn’t work, what sort of things to the readers do not like, and so on.) I also know that the reader is not always right, but the reader is always the reader and any complaint should be heard out. A decision to modify the work according to the feedback should be something that is deliberative. I genuinely want the readers to come back to me and give me honest feedback as to what stinks. Of course, those people are friends and family, they want to be supportive right? They don’t want to make critical feedback that’s going to make me feel bad.*

So today I had an exchange that ran: “So, tell me how much you loved my book so far.”**
“I only read the first two pages.”
Both of us laugh, “Sucked that bad did it?” I asked.
“No, no, I didn’t say that.”
The joking starts to cool, but I try to keep it going.
“Really, did you not like it? What sucked – I can’t fix it if you don’t say.”
“Oh, well, it was fine really.”

… This went on for a while before coffee break ended and I went back to work.

I did finally get some advice from this, and I feel like it was good and addressed some things I have been worrying about. It essentially confirmed my worry. The problem was that it was pulling teeth to get that feedback out, and I’m pretty sure I sounded like an ass in asking for it. As near as I can tell, the reluctance is that the work is a little rough, and in pointing out issues there is concern that I’ll take it badly or argue about the advice. All I really want to do is to improve, and even if I think the advice is crap, I do appreciate it. Of course, I also recognize that I’m asking people to read 95,000 words of something they may not like, and not because the writing stinks.

*I’m going to feel bad about comments describing any significant revisions.
** This was done jokingly, and appropriate in context.