People have no idea what it's like trying to lose weight.
If you are used to eating a large amount, when you try and cut down you get hunger pains, actual pain, until you eat something. You can try and hold it off yes with Lettuce/Water w/e but then once thats been used up you're left even hungrier and in more pain.
Also some people have very different metabolisms. I can be good all week, then have a Pizza at the weekends and it undoes all my hard work. But I had a friend at school who ate donuts for lunch every day and was under weight.
Also when it comes to exercising, exercising is really hard if you're overweight but easy if you are fit. It's so unfair as you're caught in a catch 22. If you're obese you can't go for a run, because you're carrying all this extra weight on your back, imagine trying to run with 25 kilos strapped to your back?
Also there's so many conflicting messages on what's healthy and what's not?
Every diet tells you X food is good, Y food is bad. Then a friend tells you that's all completely wrong and actually food y is good and food x is bad, then someone else says actually they are both bad try food z, then someone else says no Z and Y are bad, food X is good... and it just goes on and on.
Ridiculing fat people and lowering their self esteem doesn't motivate them to run up to the gym where you can laugh at them some more, it just makes them feel sad and down and leads to comfort eating.