How about I tell my friend that I can't go out to eat with her anymore?
Why can't you go eat with her anymore?
Because I'm obsessed with losing weight.
Obviously you can choose a different activity if you really need to. But obsessively worrying about your weight is also not healthy. Maybe you could find restaurants with some healthier food choices.
So you're saying I'm not allowed to make my own choices? I'll do whatever I want.
You can do whatever you want, but obsessing about your health isn't healthy either. You can still go out and just eat a salad or something, because going out with your friend is more about spending time with them than the eating.