What makes Gene realize that Finny thinks of him as an extension of himself?

It seems that Gene only really appreciates what Finny was to him when Finny dies. Finny had thought of Gene as an extension of himself, but it is only after Finny's death that Gene feels it is his own death too. Finny was the sort of person who appreciated people, like Gene, for what they were to him, and felt free to tell them their significance. Gene, however, wasn't even able to acknowledge what Finny was to him until it was too late, and Finny was already lost. This recalls Finny telling Gene that he was his best friend, and Gene not being able to say the same thing; Gene is unable to search his feelings and come up with the same conclusion as Finny.