Why does Marlee realize that relating people to drinks isn't really fair?

Marlee comes to understand that relating people to drinks isn't fair when Liz takes her to task in Chapter 53, not quite ending their friendship, but telling Marlee that they both need to look for new friends. Liz frankly tells Marlee that she is merely a cup of warm milk with cinnamon... that she is so much more than Marlee perceives her. In turn, Marlee comes to understand that her perceptions have interefered with her getting to know people as they really are.... that "most people were a whole refrigerator of different drinks."