For me, with a good bread (latest favorite is Dave's Killer Good Seed bread), peanut butter is the essence of the sandwich. Jelly makes it go all slidey and drippy. The sugar hides the natural peanut flavor.
Timothy likes jelly in his sandwiches.
I just made a peanut butter sandwich and then ignored my instincts and put a nice dollop of homemade grape jelly (harvested from a friend's vineyard) on it. And cut off the crusts and tucked it in his lunchbox.
'Cause that's what parents do.