Why does Leon only drink milk?

I think it was used to show that while he is a professional killer, he is also very innocent and naive about the world. Drinking milk all the time is something that mainly children do and since Leon has been so cut off from the world he is a sort of child mentality towards it. Like during the scene where him and portman are dressing up as characters and guessing who they are and Leon can't get any of the modern references, including not recognizing who Madonna is (she was one of the most well known people in the world at the time). At least that is what I think the milk was about.