Cuidate vs te cuidas
Cuídate = take care, look after yourself
It is like an order but as you can imagine it is a piece of advice too. I mean, in Spanish, it is an imperative.
No te cuidas. Deberías cuidarte más. = You don't look after yourself. You should look after yourself more.
"Cuídate" is not the same as "te cuidas". It has a different meaning because the former is an order.
Anyway, you can say: te cuidas, ¿eh?. The sense is the same as "cuidate". But take into account that the form is different.
Siéntate = Sit down.
"Ahora te sientas" (now you sit down). It is true that can be used for an order too, but it is not imperative.
I understand cuídate to be a command (Take it easy / Take care), and te cuidas to be a statement (You do take care of yourself).
The first is often used as a farewell, I think (I have often seen it used like that in movies, anyways).
Both are in the tú form, by the way.
So when he left and said "te cuidas/es" what did he mean? You should take better care of yourself? I have been dieting, maybe I look too thin? That sort of thing?
It depends, it can have several meanings. It can be a farewell like in English. Or perhaps it is a piece of advice.
Well, I suppose it is like in English.
You are talking to someone and when you say goodbye, you say: cuídate (take care).
But when you are talking to someone about him or her (her/his health, his/her habits and so on) you can say: cuídate más or deberías cuidarte más. (It is the same as you should look after yourself).
Yo me cuido = I look after myself
Tú me cuidas = You look after me
Tú te cuidas = You look after yourself
El se cuida = He looks after himself
El la cuida = He looks after her
(All of them are in the indicative form)
Cuida a tu hermano = look after your brother
Cuidala = look after her
Cuídate = look after yourself
(All of them are in the imperative form).