I'm not a relationship expert either. However, based on what you've shared, I would guess it's insecurity. When you were obese, perhaps he didn't worry about other men being interested in you, but he could see this as a real possibility now that you've lost so much weight. He may simply need reassurance of your commitment to him. This should be obvious after 20 years, but your circumstances have changed. Even if we know it's true, we all want to hear our partners tell us that they love us and would never leave us!