The only parents who do not practice favoritism are those parents who have single children. In multichild families, there is bound to be favoritism. When there is more than one child in the family, one child is going to be the favorite. Many factors influence favoritism from having similar personalities to that of the parent, having prodigious talents and gifts, having the same/similar outlook whether it is religion, politics, and/or other arenas, having talents that is desired by the parent, and/or other related factors.
Parents also favor children who are not deemed to be difficult. In other words, they favor children who they consider to be easy and not stressful. That is why parents tend to favor children who are obedient and otherwise compliant. Such children are easy meaning that they are manageable and bend to the parents' will. Children who are more independent and assertive are oftentimes unfavored children because they are considered to be difficult.
There are rewards to being the favored child. He/she has the perks and privileges that other children do not. He/she oftentimes have the inside information and access to the parents that other children do not. He/she is special and the golden child of the family.
However, the favored child is held to higher expectations than the other children in the family. He/she is sometimes expected to exist at the total behest of the parents. He/she feels that he/she is under undue obligations to the parents because they have "done so much" for him/her.
As the old adage goes, if one is given much, then that one has great expectations. Oftentimes, there is resentment and anger on the part of the less favored children in the family towards the favored child. Many times favoritism creates a chasm among siblings which lasts into adulthood.