You are looking a gift horse in the mouth when you receive a gift and then you question the value of that gift. Example: "This shirt you gave me is not my favorite brand." Reply: "Don't look a gift horse in the mouth! That was the best they had."

You are like a person who has been given a horse as a gift (a gift-horse) and you are looking into the horse's mouth to see if it is in good health. Example: "Here is your drink." Reply: "Oh, I hate this kind." Answer: "Don't look a gift horse in the mouth."

You should be thankful when you receive a gift, and not wish for something better. Don't look a gift horse in the mouth means don't measure the value of something that you have received for free. Example: "Great interception!" Reply: "Yes, but I almost scored a touchdown." Reply: "Don't look a gift horse in the mouth."