We generally don't want to be 'sad' or 'feel sad' but the problem is 'sadness' is apart of our life and 'feeling sad' means we are able to realize that something is wrong in our life, or we know exactly what is causing pain in our life but we failed to realize it before.
Normally when we 'sad' we cry, that is common reaction.
'Can't eat' or 'can't sleep' means the current the current issue/pain is a extremely big problem which really mess up our beliefs and values.
'Eating' is an interesting reaction cuz many people eat alot when they are in trouble. It is proved that eating somehow renders our ability to think, so eating can temporary prevent us from thinking about the 'sad' thing (but after that 'sadness' will surely return).
'Stop being sad'... this means the person finally found out the way out (or hope) in order to solve the problem.