Dreams are your brain's way of stitching together recent memories into something that keeps the subconscious busy, while you get your restorative sleep. Obviously, if you're thinking about diapers in your life, or wearing, then that memory can be stitched into a dream. It's not a hard & fast rule, and I know someone will come on telling me that their dreams aren't about things that happened in their conscious lives, but generally speaking, if you can recall a dream, and examine it, you'll probably find some recent memories can & sometimes do play a part in their makeup.
OK, so I studied dreams in college, and that's made me slightly more knowledgeable about the how's & whys. Keep a notepad right by your bed, and try to train yourself to wakeup at the end of a dream (rem sleep), and, if you can recall that dream, put it into some notes, then you can damn near remember it forever, BECAUSE you're bringing it into your conscious memories, where it can be recalled again & again.
I've had many diaper dreams, the last of which I was at some type of outdoor carnival or fair, or something similar, with my wife. She had walked off to check something, and the next thing you know, three people (two girls and a guy) had come up to me, and asked me if I was wearing a diaper. I wasn't, but before I could say a thing, I was knocked down, my clothes were all removed, and I was quickly thrown into a poofy diaper & plastic pants, against my will. I was mortified! As quickly as they had arrived on the scene, they were gone, along with my clothes, leaving me there exposed to all. My wife soon found me there, and was wondering what the hell had happened to me. I couldn't vocalize or speak to answer her, so, still mortified, she dragged me, fully diapered, off to the car, where the dream sort of broke up...
I get to keep that memory now, since I did the recall thing, and brought it into my conscious mind. It's kind of neat when it happens, but it's no surprise to find yourself in a diapered dream, since, if you're like me, you're thinking about them all the time. My wife is fairly vocal during her dreams, and since I drive with people that haven't a clue about my diaper desires, I pray to God that I'm not vocalizing about diapers in my dream state. THAT would be very hard to explain...