don't look a gift horse in the mouth
If you receive a gift, do so graciously, without voicing criticisms. The saying is attributed to St. Jerome and refers to the practice of looking at a horse's teeth to determine its age. I know Aunt Jean isn't your favorite person, but she gave you that beautiful sweater as a present, so don't look a gift horse in the mouth! A: "But I don't want this ancient car!" B: "Don't look a gift horse in the mouth, OK? You're so lucky to get a car for free!"