Why overeat and get fat in the first place

I have had the pleasure of my mothers' company over the past 10 days and I have to say I am coping quite well.  It must say something about  the place I am at at the moment in myself.  I have often found spending time with her annoying and disconcerting as many of her quirks really rubbed me up the wrong way.  This time I am accepting all of her lovingly and allowing her to be her and not letting things bother me too much.  In fact when I feel a moment of discomfort that centres around what she has said or done I stop and look at what I might be able to learn from the situation.  

Something that as has me thinking is this whole idea of why she obsesses over what she eats.  She has to be one of the most diligent and what I will call obsessive people when it comes to what she eats and how much she eats and how she keeps her body as tiny as it is.