When I was 16 I thought my ex-boyfriend “owed” me niceness because he had accepted my love when we dated.   

He didn’t.  

He owed me the courtesy two people can owe each other. Or not. 

So I understand that once the relationship is over people don’t owe their exes more than distant politeness, and may owe them less. 

What is this closure nonsense?

It’s ludicrous entitlement.  No one owes anyone else closure. It’s not a thing.  Fooey