Don't forget what we had

Finn and Millie have always had a very strong connection ever since they first started filming stranger things.
They were inseparable, they would do almost everything together.
After they finished filming season 2, finn had started to feel that their "connection" was fading. And he had an awful, haunting feeling he was about to lose what they had, their friendship. Their pure, beautiful friendship.

Millie meets a new boy and they start to form a pretty damn good friendship, possibly a serious relationship.. and finn is truly heart broken. He can see that this boy and Millie have a connection, just like they did.
Finn doesn't speak up about it because he doesn't want to ruin anything. He's scared that if he does say anything, the small remains of their friendship will be gone, forever.
--
"But, finn.. I think I'm in love with him."-
"Just please, Millie. Don't forget what we had."