Sweet and Salty

I've got a thing for things that are sweet and salty. Caramels with sea salt. French fries with plenty of ketchup. Peanut butter and strawberry jelly. Melon with prosciutto. Is there anything better to eat on a hot summer day than a ripe cantaloupe dressed with some cured meat, thinly sliced? I think not.

But why do sweet and salty sing so well together? Why do we add a pinch of salt to chocolate cake, or not fully taste the sweetness of a tomato until it's been sprinkled with sodium chloride?