The reason this sometimes work is not because it objectively should. It's because it helps the other person relate. It's way more difficult to convince someone

The reason this sometimes work is not because it objectively should. It's because it helps the other person relate. It's way more difficult to convince someone of something if they already see you as "their ideological opposite". It's a chasm too wide to cross for most of us humans. The "gaining weight" conversation is something that they understand, probably have experienced themselves, or have seen their spouse try. It's not something that can be perceived as coming from the "scientist caste" (this is a real term I have heard used). You are less of an opponent and more of a next-door-guy trying to explain something. It's a bridge that you lay over the chasm. They still need to walk over it, though. I'm sure there's some sports argument that can be laid out in a similar fashion.