Forbidden Friendship

A character wants, very badly, to be friends with someone specific. But sadly, the rules of family or society look down upon the friendship, and won't allow it. There's a number of possible reasons for this: If their relationship is or becomes romantic, they may be Star-Crossed Lovers.
 * Prejudice. Whether it's class differences, racism, or any other difference, an entire group of people is declared "evil" and must be avoided if one is to be socially accepted.
 * Personal vendetta. Sometimes one family hates another, and won't allow their own to hang out with the other family. Not everyone in the family shares that dislike, however, and they might become friends anyway.
 * Safety. Sometimes the society or family has a fear of a specific individual, whether warranted or not. And who wants to see their friend hanging around someone who might kill them?
 * Bad influence. Sometimes the specific individual isn't so much a threat to the character, as to society's value system. Friends who do drugs, get into trouble, and so on, might be disallowed as company for the main character.