I would think it's about "balance", kind of like dreams where we often will dream the opposite of what's happening in life because we're getting too set in what's happening. If you are obsessive about being "healthy" and "lean" (both your definitions, it's your body so there is no normal, average, correct, comparative definition) then your mind and body will occasionally remind you of that and that you're leaning all one way instead of "balanced". Theoretically, since our bodies are actually whole, they will naturally do things the way that is good for our whole selves if we're balanced people. Feeling like our body is not part of "us" and is something to be battled and forced into a particular shape, etc. is going to cause a rebound.