My view is that simply finding oneself in such a dialogue with a SAD person is a no-win situation. I consider myself successful when I am eating the way I want to eat, without providing any form of explanation or justification or rationale. Without even discussing diet. If this means my non-eating of cupcakes captures the attention a serious pastry eater, at most I'll find myself simply saying, "No thanks" and refusing subtle invitations to argue.
Imagine that you're a pagan, or an atheist. And you decide to go to the local Southern Baptist congregation where you think it might prove fruitful to make general statements in behalf of the tenets of your pagan or atheist worldview. Would that be a conversation you'd like to have? Would you expect to be greeted warmly and kindly?
People very rarely switch paradigms based on evidence. In the present case, this is because the evidence that a Paleo eater produces (studies, research, science) is ipso fact considered invalid at worst, irrelevant at best.
Now, if someone I love or care about is eating SAD and asks about the diet I seem to be on, I may choose to "share" but only in a setting that is free of contention or the need to be right or to make anyone else wrong. The key is: I need to receive an invitation to talk about my diet, before saying word-one. Barring an invitation, I simply notice that my diet is different from yours, or hers, or his, and that's about it.