Explanation: To be “pampered” is to be taken care of in a very nice way. Someone might go to the spa to have her nails done and get a massage and we could say, “She is being pampered.”
If you are too pampered or if you expect someone to always take care of you we could call you spoiled. “Spoiled brat” is a common phrase is to refer to a child who gets mad because he doesn’t get what he wants.
Knowing that we get milk from cows we can make the joke that pampered cows give us spoiled milk. The joke is funny because spoiled milk is what you get when you leave the milk out of the refrigerator for a long time and it turns sour, it spoils. It doesn’t come from cows who are getting their nails done at the spa.