And if you are at a BBQ, and the host is providing food, well.... it kind of IS their business what you eat, since they are feeding you. The reasons why aren't anyone's business either, but then responding in a weird, possibly over-reactive way about it will only get odd looks. It's possible that when someone asks why, it's just a conversation starter.
I said this already actually:
Do note, I'm not necessarily giving a full accounting of the conversations I have in real life, where I would share at least a summary of the reasons.
Did you not see it, and apply that same reasoning to the what I said in the paragraph above? Would it have helped you if I had? Can you apply that principle now and stop assuming what I'd be saying in real life? It would be much appreciated.
There's no real BBQ. I don't feel a need to give a specific reason when this is just some random person on the internet who I'm not actually eating with. Since some people want reasons, I gave a list of them, any of which I'd think would be acceptable enough, without specifying my own, because you know what I didn't want to do? Justify them further. I'm sorry but I just don't consider that to be especially desirable at this point in the thread.
It's possible that when someone asks why, it's just a conversation starter.
And it's possible that when somebody doesn't give a reason, they're just not interested in having that conversation. Which may be for any number of reasons. Would it help you if I listed some of them?