After some attempts at interacting with Greg, the way to his heart does seem to be through his stomach. If you can get him talking about food, or even manage to have a meal with you, he becomes a lot more personable and cheers up considerably.
A Celestial Crystal Crown fitted with one of the appropriate crystals can help a lot as well.
The result of him crossing paths with a werewolf toddler also seems to suggest that he's coded to avoid interactions with children, at least while he's in a bad mood. He actually turned and ran away from the toddler son of my werewolf couple, after making some frightened gestures, which is rather funny. I suppose even Greg would be horrified with himself after considering being mean to a small child.