The Calendar Is Not About the Calendar
When you add his dentist appointment to a shared calendar, you are not being organised. You are saying: your life is inside mine, and I am holding some of it. That is the whole thing. The colour-coded blocks and recurring reminders are just the surface. Underneath is the agreement that neither of you will drop the other's obligations, that neither of you will pretend a bill, a parent's birthday, or a car service does not exist because it was not their idea to schedule it.