For me, it's very simple. While I enjoy a crunchy vegetable sandwich and a good juicy burger, from time to time, the sandwich that has my heart is the tomato-mozzarella hot panini, Italian-style. Hot and crusty, bread compressed into crispy toast, creamy mozzarella and tomato juice oozing out the sides. If I had to eat just one sandwich the rest of my life, that's what I would choose. Of course, how depressing would it be to give up all the range of sandwiches, hoagies, and subs for just one variety! And yet, we each probably have a sandwich that could hold us, if we had to make a choice. A sandwich we cling to out of love and familiarity.
There are also the instances of pickiness or set routine. My brother ate nothing but peanut butter and jelly sandwiches for a brief period in his toddler years, and my father subsisted on a daily Big Mac during law school. Do you have a sandwich like this? Your go-to meal?