"Human beings are like milk. Some are churned into sweet butter and some become whey. Why should some always be lucky, be born to a high station, and never experience sorrow or want?" (The Cripple.39)
This is one of those metaphors along the lines of "Life is like a box of chocolates, you never know what you're going to get." The quote suggests why some people are born into a life of ease and general awesomeness, while others always have to struggle: that's just the way things are, some people are rich, and some people are poor. We don't think being churned into butter sounds any more pleasant than being made into whey, but, hey, whatever works.