I just answered a question on ELL.SE that made me (and the OP) wonder about something... Consider the following four question responses to these two statements:
1- "I know I'll marry young."
2- "I'm sure I'll die young."
A. Why sure?
B. Why so sure?
C. How sure?
D. How so sure?
My thoughts on the questions themselves:
- B is correct and commonly used, but D is incorrect.
- B and C are both grammatically correct, and both ask about the degree of certainty, but B asks about the reason for this degree of certainty, while C asks about the actual degree of certainty (and not the reason behind it)
- A is technically correct, but would usually be used as Why "sure"?, and sounds awkward with the meaning of Why are you sure?
My thoughts on the questions as they relate to the statements:
- For statement #1, only B is a suitable reply.
- For statement #2, not only B, but also C and a modified A (Why 'sure'?) are suitable replies.
Why is "Why so sure?" correct, but "How so sure?" incorrect? Is it just a matter of fixed expressions, or is there another underlying reason?
Why is "How sure?" correct, but "Why sure?" (without the quotations around sure) sounds incomplete?