Most families can't finish an entire meatloaf in one night, and that's a good thing. The dish contains so many ingredients that meld together as it cools in the refrigerator. The liquids that release from the meat during cooking start to solidify again as it cools, creating a sort of gel that catches the flavors of the other ingredients in the dish. Since meatloaf uses ground beef, there's even more surface area for the gel to form and collect flavors. When it's time for dinner the next night, the reheating process releases even more flavors, especially from aromatic ingredients like onions and garlic.