A Sweetheart (known as Special Someone in the European version) is the person who becomes a boyfriend/girlfriend after a successful confession. Various problems can cause Sweethearts to breakup.
Miis may want to breakup are if they argue a lot or their relationship status is low. After a breakup, both people in the relationship may become depressed. If the relationship status remains "Let's Get Married!" for long enough, one of them may want to propose. If they used to be best friends, they won't fight.
Gallery
Two sweethearts playing tennis together.
Two sweethearts at the beach.