Does milk seperate or settle even though it is good?

Ever since i was a kid, whenever I am going to pour a gl of milk, I shake the jug around a bit because i think milk seperates, so I shake it to mix it up... My wife thinks this is ridiculous and milk does not settle.. If the milk settles, then it is bad, and it is time to throw it out.. so does milk just settle, or does it settle when milk is bad?