Often the questions we ask about our kids are borne out of our reality, our perceptions........asking why, how, what, or any question from our framework is usually a futile and frustrating experience which leads nowhere. It's of course, an absolutely normal response to abnormal situations, however, there usually are no concrete answers which will offer understanding which is what we are seeking. And, as parents, I think we are also looking for somewhere to put the responsibility or blame, is it us? Is it a disorder? Is it because of something I did or didn't do? Is it substance abuse, mental illness, a personality disorder, what is causing my child to act in this way? There has to be a reason. When our kids are young and we are responsible for helping to 'fix it' whatever that requires, it's different, we have power to make a difference, we have some control, we can ask those questions and seek the answers to get the help our child needs. But once they are an adult, once they refuse all help, once they are making their own choices, asking ourselves 'why' can keep us stuck for a long time, spinning around in the insane world where we have no power or control to change anything, trying to "figure it out" when there really is no "figuring it out".
I asked all those questions too. The need to find out why is compelling. After doing that for awhile, I started to see how I may never know, I may never have a reason. I still don't. What became important is that I change my responses, I change my perceptions, I change. I put all my energy in to that. It was definitely a learning curve, a process, it took time. Each step of the way, I had to shift gears and learn a new way to respond.
All of those dialogues with your son are crazy making for you, or for anyone. They are circular, abnormal conversations which lead nowhere........designed to keep all responsibility away from his doorstep and placed on any other doorstep. Trying to understand is an exercise in futility. Let it go. Keep your interactions with him to a minimum. Don't engage in crazy behavior, or you too become crazy. Step back from the edge......... it is very easy to fall in the abyss from the edge.
I have never figured out what the cause is of my daughter's behavior. I have a bunch of guesses, but I don't really know. What I do know is that I had to remove myself from that sphere of influence and learn how to set very clear and unbreakable boundaries. That is what helped me to find my way out of the insane part of all of it. As I did that, our relationship improved.
You're doing a good job. Stay the course. Focus on you and what YOU need. Keep yourself well supported.