You know that I don't want to eat it. I've been dieting for the past 2 months, and I've had some great success in losing weight because I've been disciplined with myself. It's been very hard on me mentally and physically, and the last thing I fucking need is everyone around me who knows that I'm dieting offering me food thinking that it's funny that I have to restrain myself from eating it. And sighing and rolling your eyes after I decline your offer doesn't fucking help the matter either. Leave me alone if you're going to be a dick about it. I don't understand why it's so hard for people to be supportive rather than destructive. This is something I want for me. Try to respect that.
If you're sensitive enough to be this pissed of by people being generous then it seems more likely you're dieting out of social pressure to seem more 'cool' or fit in better. If you make a thread like this on a forum that a decent number of members stem from 4Chan you're either troll bait or the type of person some of us target.
Frankly it sounds like you're an ungrateful bastard who picks to starve themselves rather than pick up regular and intense exercise to stay healthy. Most of the time I don't have enough money to buy food at lunch or that I can bring there, so when someone is willing to give up some of their lunch for me I'm damn thankful rather than jumping to assume they're messing with me.
Now that doesn't excuse them from being pricks, if they are doing it to mess with you they're obviously getting a reaction they find funny or worth the effort of messing with you. So stop being so fucking sensitive and don't React™. The joke should die out once you stop caring so much.
tl;dr Quit having the attitude of a 3rd grade school girl and be able to run a well paced 5k in a month rather than being a picky eater or GTFO