In Season 4 Episode 5 of The Magicians ("Escape from the Happy Place") it is seen that Eliot has to go through his greatest fear and sad memories to unlock the door. What exactly was his fear or memory? He meets Quentin and says that he gives him the strength to face his greatest fear. What was that fear and what was Eliot feeling at that time?
That scene is an extension from the episode in Season 3 where Quentin and Eliot live 50 years together to find the true meaning of life so the key will reveal itself (A Life in the Day).
Once they get the key, they teleport back to the throne room which is where Eliot faced his greatest fear. Quentin reveals his love for Eliot and suggests they actually have a real relationship together. Eliot has had friends, but always kept them at arms length. He never had a real, serious, intimate relationship with anyone and the idea terrified him.