When you make your own sandwich, you anticipate its taste as you’re working on it. And when you think of a particular food for a while, you become less hungry for it later. Researchers at Carnegie Mellon University, for example, found that imagining eating M&Ms makes you eat fewer of them. It’s a kind of specific satiation, just as most people find room for dessert when they couldn’t have another bite of their steak. The sandwich that another person prepares is not “preconsumed” in the same way.
A large part of tasting is actually smelling. You can verify this by, for example, eating an onion with your nose plugged. Our sense of smell tends to filter out persistent smells after being exposed to them for awhile so that we cannot smell them anymore. This means that when you are cooking in the kitchen, surrounded by the aromas of your food, you are quickly de-sensitised to them. Then when you sit down to eat, it is like tasting without smelling.
When your spouse has done the cooking you were likely in another room, isolated from the aromas. When you walk into the kitchen to eat, you get to smell and taste the food at the same time. That’s why it tastes better to you. The same idea applies to leftovers. It takes much less time to reheat leftovers than it took to prepare the food in the first place so you retain sensitivity to more of the aromas when it comes time to eat.