I’m glad you and your little Green Cheek are such good companions. To understand his behavior, you have to understand wild parrot behavior. In the wild, young birds will live in a flock, but once they are mature and have a mate, they do not interact with the other birds. Any bird that gets too close is a rival, and will be attacked. If the rival cannot be reached, the male will often attack his mate to drive her behind him, or into the nest if it is nesting season. So this is what your bird is doing. Often this is hormonal and will only happen some times. But he may not even accept other people anymore. There isn’t much you can do other than to put him in his cage before someone comes over. It is natural behavior and trying to break it will only break his trust in you. Keep him as your companion and explain to friends why he does not want them to handle him.