On so many levels this scene kills me. It’s an intimate scene with the Arrow Trinity and on the top level it’s Oliver pushing Diggle away because he doesn’t want to be responsible for getting John killed before he gets a chance to be a father. So many times, Diggle is the voice of reason for Oliver - protecting him, keeping him human - he understands Oliver’s soldier perspective, but this time Oliver sees the need to play that role. It’s really an incredible moment for his character development - the entire episode he’s toyed with being the two sides of himself (paralleled by the flashbacks with Maseo ‘a man cannot live by two names) only to, ultimately and for himself, choose to be only the Arrow. He makes that choice and then makes the decision that no one else should be forced to live like that - because he despises that he has to do it and so he pushes John out.
And then, in the middle of that emotional scene - right in the middle of the 'you have a new life’ and 'it’s a life I can’t have’ line - Oliver distinctly looks towards Felicity, who is studiously trying to keep it together even though the subtext is all at once subtle and blatant (to Felicity) and heartbreaking. He’s no longer looking at John when he delivers the 'It’s a life I can’t have line’ and the camera makes the parallel obvious when it cuts to Felicity shifting uncomfortably and Oliver cuts back to Diggle - cutting the cord in a bit of an angry argument, taking it as Oliver has so many times before and pushing down the sadness he has to be feeling.
Because Oliver knows - Felicity knows - that the next person up is Felicity and that one will hurt much more because the revelations are so new and that scares Oliver almost more than Diggle risking his life - because he knows if he lets himself be Oliver Queen and the Arrow that one side will suffer. And he’s terrified that that side is The Arrow, then Felicity will be in even greater danger - he was angry with himself after their date because his carelessness and willingness to be Oliver Queen for FIFTEEN MINUTES ended with Felicity being hurt.
And as Diggle walks away from the trinity, Oliver is staring off trying to keep his emotions reigned in and you see Felicity crying in secret.
She’s crying for both men - the loss of John, her friend - and Oliver Queen, the love of her life.