There are two kinds of favorites: the harmless and the insidious.
The harmless are easy enough to recognize. My favorite flavor ice cream is chocolate, because I'm a bland and boring person and can never get enough chocolate anyway. I don't have a favorite color because I love them all and it feels like discrimination. These favorites are harmless, even cute when they aren't carried into the realm of obsession. These favorites don't hurt anyone.
There is the other kind of favorite.
I call these favorites insidious, because to have this kind of favorite does not make you a bad person. I know many (okay, one, but there are more out there, I'm sure) people who manage to be basically good and still keep a favorite like this around.
I'm talking about step-daughter favorites here. Like- exactly like- Cinderella: The step-daughters were the favorites, and she was... not. I'm talking about a boss favoring one worker over another for no reason at all. I'm talking about a mother loving her oldest, or youngest, or middlingest child more than the others. I'm talking about 'best friends', where you love one or two of your friends to the outright exclusion of others.
Playing favorites does not create a loving or kind atmosphere. It is unhealthy for all involved. The person playing favorites is taking the stance of having the right to judge who is better than the rest. The favorite is encouraged to believe that they are somehow the best, just born that way, and have a god-given right to get away with more than the rest of us. And the people who are not the favorite(s)...
Well. It makes it very hard to love them anyway. And you can't, usually, just explain how you feel: "So-and-so, I think that you love him/her more than you love me, and it's putting a strain on our relationship, blah blah blah". Because the knee-jerk response of the favoritist is "You're wrong. You're a horrible/misled/stupid/(insert other unfavorable label here) person. I would never play favorites. I love you all just the same," as the favorite gets away with bloody murder on the other side of the room.
Like I said, there are two kinds of favorites. The first kind is harmless. The second kind is insidious, because it takes a very honest, even brutally honest person to see and acknowledge to themselves what they're doing. And those kind of people seem to never have favorites.
The next time someone asks me who I like more, them or so-and-so, I'm going to smack them. Well, probably not, but I'll want to. I always have.