PBJ is the only lunch food that doesn't spoil when you are out in the farm fields. Us kids got to work in the fields beginning at 10-12 years old. I got started at 6, and Mom came from a wealthy family, until the War took it all.
Personally, I prefer my peanut butter and jelly separate. I apply peanut butter to one piece of bread and fold over. Then I put jelly on the other piece of bread and fold over.
While I will eat the ingredients together, it isn't my preference. I have no idea when I started doing this. Probably when I started making my own lunch in elementary school.
I do remember that the oozing of stuff and soaking through bread always annoyed me, and I hated having sticky hands at lunch!
It may seem weird, but I don't think there is any underlying component there, except fear of stickiness.
