Favoritism is one helluva drug, and sadly a lot of people are addicted to both giving and receiving it. When you're on the inside of a favoritism bubble it feels so good, you know you've got a step up from others and that provides a sense of security.
But when the shininess of favoritism is wiped away it reveals frustration and inequity for all other parties involved. We all carry bits of favoritism towards loved ones and preferred coworkers, but ideally that comes out in the context of private relationships and not at the expense of others.
Unfortunately, there are plenty of examples of obvious favoritism, as evidenced by a popular Reddit thread full of examples.
Way back in elementary school, I had three classmates ("Kim", "Kelli", and "Kristi") whose mothers handled all of our classroom parties. We always had an egg hunt around Easter. The three students who found the most eggs won chocolate bunnies. We had about thirty kids in our class, and all of us were to bring in three eggs to be hidden. The teacher would pull down the window shades so we couldn't look outside to see where the moms hid the eggs on the playground.