I can disassociate at will, although I have no memory of any childhood trauma, so I have no idea why.
The only time it is not at will is if I am talking to someone, usually someone older than me or in a position of respect, I'll zone out and not be able to zone back in. I'll just keep nodding and smiling but my vision won't focus and I get freaked out that the other person is going to notice I've zoned out.

Now for the most part, it is at will. When I am really depressed, it is always at the edge of my consciousness, like a threat that I could slip out of it at any time. And slipping out of it feels like I am not me. For me it's always when I'm alone, and almost always when I'm in bed at night, thinking. I will start thinking about the world and my life and, suddenly, I am not myself. I cannot be (Firstname Lastname) because that would be..crazy. It feels like my normal day-to-day life was a fun little peek into the world and now I should go back to being my real self, which is ??? not sure, some type of otherworldly, floating spirit maybe? In my head, I just cannot fathom the idea that this is all there is, that when I return to my body I will be myself for the rest of my life. I think "am I really worried about X, Y, and Z? that's what my worries are? am I really going to do A B C tomorrow?" Nothing about me feels like mine.