I think the door is the opposite to suicide. It means you are in peace: as a human, you've reached your peak, the human Nirvana. Time to become one with the universe.
I saw a reference to a short story called The Immortal, by Borges. I loved it when I read it, back in the day. It's about a guy who is looking for a race of immortal humans. He thinks they will be wise and admirable, but when he finds them, they turn out to be like lazy beasts. Because everything they could learn or do, they could do it in the future. Without time, there is no purpose, no meaning.
Anyway, I loved the episode. Didn't cry, but it was sweet. I'm glad we saw different paths and so many old friends. I'm going to miss this show a lot. Its humanity and optimism are amazing.