I just happened to have one when I was little (don't remember exactly when the first one would be; the earliest one I remember was when I was about 10). Not because of nightmares or anything, they just sort of happened for some reason, maybe once every couple of years or so. I always assumed it was normal to have LDs occasionally, and it wasn't until much later that I knew they even had a name.

I learned a bit more about them during my teenage years when I read about them from a book, and had a brief burst of them around that time. I don't recall doing anything special, just happened to have it on my mind a lot and would up with some DILDs. That's pretty much how I started having them more often several years ago when I really got into them and found this forum.