John has been confessing his feelings for Harold since season 1. Every time he says “thank you for giving me a job”, “someone found me and gave me a purpose”, he meant it with all his heart. But it’s really what was left unsaid that made his statements mean more than anything else.
This though, the way John looks at him, the way he emphasises you to Harold— for me, this trumps every other moment he professed how he felt because for the first time ever, there isn’t anything left unsaid.
So I don’t think it’s wrong to say that this is possibly the closest thing to John saying “I love you” to Harold and now I’m a fucking mess.