A text exchange with DP reminded me that our reactions to this are completely different.
For context: DS2 has just had his vaccinations and is feverish, irritable and generally a pitiful state. I'm back at work (WFH) and have spent the night breastfeeding DS2, DP (on shared parental leave) has spent the day trying to rock DS2 to sleep. No one is their best self.
DS2 is finally in a fitful sleep (on DP) when the doorbell rings. I answer (because of the sleeping baby) and have a conversation with a stranger about how no, the car with the window down isn't ours and no, I'm not sure who's it is but I know it's definitely not our immediate neighbours. They leave, I go back to work and get a text from DP asking what's going on and how DS2 almost woke up and if I'm on the phone I need to have the door closed. I explain and think no more of it. DP says that they wanted a "sorry". I apologise and everything is fine.
But the thing is it didn't occur to me to apologise, and I don't see why DP wanted one, and this is a semi frequent thing. I apologise if I've done something wrong (for instance, I would have done if I'd been on the phone with the door open) but I don't when there's no fault at play. And I find it bizarre that DP sees it as just standard politeness to apologise!