Because going on a diet implies that at some point in the future you will go off the diet, and if you go on a diet and lose weight and then go off the diet, and if nothing else has changed in your life, then the weight you lost while on your diet will most likely return. With friends.
In order to lose weight and keep it off you need to effect a lifestyle change.
Instead of going on a diet you need to change your eating habits. Instead of eating three pork tacos for lunch, you'll eat two. Or perhaps you'll eat two chicken tacos, or two fish tacos.
Instead of riding the bus to the office, given the time you'll get off several blocks early and walk the rest of the way, and in the afternoon going home you'll do the same.
And if you want to keep that weight off those are not things you'll do only until you've lost X pounds, after which you'll resume eating 3 pork tacos for lunch while riding the bus all the way to the office. If you want to keep that weight off you need to continue doing the things that helped you lose the weight in the first place. In other words, you cannot go on then go off your diet.
That said, for the past few months I have been on a diet. I make no pretenses about this. I am on a diet. I have been denying myself certain food products in order to lose weight, and once a certain date has been reached, being October 5, I will absolutely and unequivocally go off my diet. After October 5th I will have a donut from the box that occasionally appears in the office. One donut. I will sometimes order the ribs instead of the fish or salad when we eat out. And I will have those deep fried jalapenos, the crispy fried noodles, and the pork burrito, thank you very much. But not all the time.
Because once October 5th arrives my Office's weight loss competition will be over, and hopefully I will have lost a large enough percentage of my starting weight to be declared the biggest loser. That's my primary goal. To lose enough weight to win the Office weight-loss competition.
Currently I'm the front runner, as per our weigh-in last week and now there's just 3 weeks to go. I weigh approx. 175 lbs, having lost 12.2% of my starting weight of 200 lbs, I'm wearing size 32" pants (which fit well, and are not snug at all), while my body fat is approx. 15% (it used to be around 22-23%). I'm in great shape for a 42-year old guy. Knee pain & arthritis aside I'm even in better shape than when I graduated from High School. So I can honestly say that once this is all over, even if I'm not the biggest loser, I am a winner.
However, just because I'm going to go off my diet on October 5th doesn't mean I'll balloon up to my former weight of 200-210 lbs. Yes, I'll have the burgers and ribs and beer and jalapeno poppers, and all the other foods I love but which I've been denying myself these past few months. But I'll eat them in moderation. I said earlier that winning the Office weight loss competition was my primary goal. My secondary goal is to maintain (to a moderate degree) my healthier lifestyle and to continue working out. But rather than continue with the cardio I've been doing to lose all that weight, I'll be lifting weights to regain the muscle mass I know I've lost.
In 3 months I've lost 25 lbs and dropped my body fat from 23% to 15%. I'm not going back on another diet; I'm going to make changes to my eating and workout habits in order to capitalize on what I've accomplished these past few months.
I don't plan to bulk up and look like Captain America, but I still want to look good.
Correction: I'm already looking good, I want to look great.