Sometimes it's odd, but after you have experienced something - something very dramatic like the feeling of depersonalization - it's hard to forget the feeling.
I remember one time, when I was young, I was playing around in the backyard and accidentally took a baseball to the face. It _really_ hurt like hell - and, I was about nine at the time, I remember waking up a couple nights with terrifying nightmares where I was getting hit with the ball again - the pain was terrifyingly real. I would wake up, and, no pain - but it was extremely real in the dream. Odd story - but true.
Sometimes when I sit and think about DP, it comes back - simply by thinking the feeling again (yes, I did mean to write the sentence that way). If you had depersonalization at a time in your life - I have a strong feeling that it was strongly associated with anxiety or the extreme feeling of "got to get out of here". It was very dramatic to us - again, my thoughts on why it's so extreme to us apply here - and we remember it and can bring it back through our normal ruminations.
Once you've walked the path, it's hard to unwalk it. Time has past and the wounds have been made.