This is probably the stupidest thing ever hypothesized by a fan. But listen.
Episode 1x05 - Elena and Stefan are in the kitchen. Stefan is cooking for her, telling her all about his life. He said one of his favorite books is “The Great Gatsby”, which takes place in the 1920’s.
Stefan and Klaus were friends in the 1920’s. Even though Klaus made him forget their friendship, there was probably a little memory inside of Stefan that makes him appreciate the time he spent with Rebekah and Klaus and all the fun they had together. He may not have remembered who he was with, but he knew that during that time, he was happy. The book probably brought back the happiness that he had forgotten.
End speech.