You have grown in his eyes during the time you spent with the Team Prime, besides, you were his human partner. Before leaving on the mission to the core, Optimus presented you with the "key to the Ground Bridge power supply" for safe keeping. The humans all watched the Autobots and Megatron step through the Ground Bridge into Earth's core. After their departure, Ratchet told about Optimus back when he was Orion Pax, and how he got the Matrix of Leadership. Ratchet revealed that the key Optimus had given you was actually the Key to Vector Sigma. When the Autobots returned, it turned out that Optimus had amnesia and he left with Megatron. When it was determined that accessing Vector Sigma would be the only way to restore Optimus's memory, you agreed to do so. You had to go to Cybertron, and with the combined efforts of the entire team, you managed to do this, along with Arcee's escort and help, already on Cybertron. With the Key fully charged with information, you and Arcee returned to Earth. While Arcee and Megatron faced off in fight, you had the opportunity to return the Key to Optimus. He was hesitant, still doubtful: "Are you sure I'm worthy?"