Main Character Attractiveness Disorder is a mysterious and swift-acting disease that strikes the majority of main characters from any piece of fictional work. If they are young and attractive, then they will inevitably be paired up, by fanfiction authors, with anyone else that is ever mentioned even once in canon, with no regard to the other person's age, gender, familial relationship, sexual orientation or connection to the character. If they're in the canon, then a fanfiction author will pair them with the main character.
This usually applies to male characters, such as Harry Potter, Kurosaki Ichigo, and Uzumaki Naruto, but can also strike female characters such as Fujioka Haruhi of Ouran High School Host Club. Non-humans are not exempt from this rule either (see Redwall and Sonic fiction for examples of this), and it isn't uncommon to see a human paired with a non-human on certain fanfiction repositories.
Not all of the characters affected by this disease are young, though. In cases where characters are immortal or extremely long-lived, it depends on how young the character appears to be. Examples of this are Captain Jack Harkness of Torchwood, Legolas of The Lord of the Rings, and Duncan McLeod of Highlander.
Another name for this disease is 'The Hero's Curse', as the afflicted is nearly always the hero of the story that he's in.