- Why, I'd love to.
- Why, of course!
I get the concept of starting a sentence with a word not completely related to the overall response, but this one seems to be a particularly odd choice — is there any semantic meaning I'm missing?
According to the OED, the use of why as an interjection dates back to the 16th century:
The OED doesn't explain why it is used in that manner. I can only speculate. First it was just a question expressing doubt reduced to its essence:
This developed through rhetorical question into a mere interjection of surprise:
From there its function of calling attention (if that's what it is) to a following statement developed:
The OED notes that it is often used this way in opposition to a vague doubt or apprehension.
Why, that's what it means!
It doesn't actually transform the sentence into a question; the question mark does that (see also: French).
We've just gotten used to always seeing it at the start of a question, but it's really there to draw attention.