Favoritism is a sad but common part of many family dynamics. Being the child who their parents didn't favor can lead to a lifetime of striving to please people and feeling like they'll never be good enough. There's enough competition at school and work that there's no need for it in families. Competing for the basic feeling of being loved can negatively affect the competitors, yet some parents facilitate it. There are so many reasons a parent could favor one kid over another, and almost none of them are anything the child can control. Sometimes, parents wanted a different gender for their baby or didn't like the number of kids they had. Maybe they're judgemental about a child's appearance or achievements outside of the family and prop up the child they see as superior in those categories. The 15-year-old in this story was favored because of the medical conditions she had as a young child, and it's ruining her relationship with just about everybody.