Bilingual thought.
I have a habit of slipping into the formal form with my dear friends, as a mark of love, because Usted vs. Tú has tonal range, and can often go full circle to intimate respect.
It's something I haven't quite fixed in 6 years (though I'll always catch myself and go back to Tú), but I think it happens because my native-English brain *loves* this range of respect/adoration we don't have in our own simple "you".
...But we DID once, with thou vs. you!
Should we bring it back? 🤔