This is interesting and a nice conversation, thank you.

He talks about how they wanted to let people know that they would stop sending them notifications after five days of inactivity, but that the "passive-aggressive" nature of that notification actually got people to come back. To me it illustrates that it's such a fine line to walk if you want to respect the user but also maybe push through their own lack of motivation.

(I'm not a user of Duolingo so I can't speak to where they land on that but it's clearly controversial)

1250 day streak on Duolingo.

The funny passive-aggressive communication style is something I personally consider Duolingo's thing. I kinda like it that they have a persona and stick with it in all of their communication.

If it was cold and to the point "you have missed today's lesson", I wouldn't come back.