When parents focus more love and attention on one child, all the children begin to feel that their parents' behavior is unfair and unpredictable, which creates resentment and uncertainty. It also affects sibling relationships, leading to higher levels of anger and aggressiveness. The less-favored child carries around feelings of not being good enough, wondering, ‘What am I doing wrong?’ This leads to low self-esteem, anger and acting out for attention, even negative attention. The favored child will begin to feel his/her sibling's resentment, and may even begin to hate being treated as the special child. (http://drphil.com/articles/article/530)
As a teacher of thirty-seven years, I too faced the parental dilemma because some kids need something more and others seem to deserve something more of us. I remember three students in an early-morning class. One, whom I will call Jane, was never late, rarely absent, and always prepared. She smiled every day, and she befriended every other student. She even offered to drive out of her way to pick up another student whom I will call Jack. He had no car in spite of the fact that he was the sole support in his household and worked more than a 40-hour week.