"Why don't we rest?"
Translation:Γιατί δεν ξεκουραζόμαστε;
The verb is passive in form: ξεκουράζομαι "to rest".
So the "we" form is ξεκουραζόμαστε.
thanks for your answer, but it is not what I want to know. There is the active word : ξεκουράζω / with the meaning I have a rest. why don't we use it?
As far as I can tell, the active ξεκουράζω is not "I have a rest" but rather "I rest [something" -- e.g. "these glasses rest my eyes" or "I want to rest my mind a little" but not the intransitive "I rest, I relax", which is the reflexive/middle voice ξεκουράζομαι.