Student or Learner
1. I would like to know if the next expression is formal or informal.
Why don't you see a doctor?
2. I would like to know which is more formal ( or informal ).
a. Why don't you see a doctor?
b. You should see a doctor.
Even the most polite structures can be made to sound sarcastic and/or impolite with the right intonation.
With the right intonation, we can make both these "polite" forms sound like the following [or something similar] are attached,
Why [the hell] don't you see a doctor [you dummy]?
You should see a doctor [you idiot]