I would like to respond to "be yourself" with "what do you mean by that?". On a literal reading, it's meaningless, because i can't help but be myself, so it must be a metaphor, but it's not at all clear what for. I honestly doubt that people even know what they mean by it themselves.
If they do manage to explain it, the obvious followup is probably "why do you think that's a good idea?".
I often found that "be yourself" really meant, "be yourself, but not like that." Or to put it more politely, "be yourself" equates to "Be the person I see you as".
If they do manage to explain it, the obvious followup is probably "why do you think that's a good idea?".