Why do they do this? [I Am Trying to Lose Weight]

Often the very people that nag you about losing weight are the ones who walk into your house with a pizza with all your favorite toppings on it or a big box of your favorite pastries two days after you finally start your diet. They bug you to eat the very foods they're telling you not to eat. They argue with you saying things like, "Just ONCE won't hurt!" Or, "Come on, it's a special occasion!" (Ground Hog Day?) Or my favorite, "It's Jane's birthday and we are all having a piece!" (As if calories don't count if eaten in a group setting with social approval.) Why do the very people who lecture you about your weight do everything they can to throw you off your diet when you actually attempt to lose the excess weight?