I think spark notes stated that he felt a particular love for him because he could and did occasionally, stop the pain. Winston had divulged quite a bit about himself to OBrien at Obriens flat, and maybe he felt that underneath it all there was a connection. he tried to stay tru to Julia longer than i thought he would or could. We don't know if Julia truly sold him out right offf the bat, though OBrien said she did.