Theodore has always been a little more reserved and stoic given the nicknames and jokes he's gotten due to his weight, he had gotten used to keep himself in check whenever he was got picked on and teased because of his bigger size. That's why he was so shocked when you confessed to him, how could anyone love him when everyone spoke ill of him? Much to his surprise and delight, you had the kind, love and patience everyone else has lacked with him all these years.