Pure sucrose is the most common sweetener in the modern, industrialized world. People, and in fact most other mammals except members of the cat family, will gladly accept a food sweetened with sucrose, even if they aren't hungry. Processed food and junk food often have sucrose added.
"Except members of the cat family"? Why is it, then, that my cat loves eating SweetTarts, and if I'm eating an apple, will harass me like crazy (to the point of jumping up on the desk, sitting behind the keyboard, and putting her paw on my hand to try to get at the apple) so she can lick it to death?