Because the constitution specifically limits certain rights, and outside of those specific limits the government shouldn't have the right to tell me what to do. Its why I'm okay with a texting or dui law - texting while driving has the possibility to impact another person if you hit them when you aren't paying attention. Its why I'm okay with limiting the rights of smokers to smoke inside where their smoke can give me cancer, but I support their right to smoke at home or outside, because it's their life and it's their choices.
Because where do you set the line about what is good for someone? Should we only allow salads or wraps from our fast food chains? Should we only allow ergonomic chairs? Should we only allow one hour of TV watching a day because being sedentary is bad for you? Should we require 1 hour of exercise every day, documented, before a person has access to the health care system?
Because telling someone that "this" is okay, but "that" isn't leads to an erosion of personal liberties, like privacy and freedom. For instance, the courts have decided that it is okay to listen to transcripts of my phone calls if I am talking to someone overseas that is a suspected terrorist or those that associate with them (the recent NSA debate). I find this to be an erosion of personal privacy and freedom.
Telling me that I'm not allowed to smoke weed (which, for the record, I've never touched and have no real desire to) because its bad for me, what does that lead to? Being told I can't have a medical procedure that I want? Which leads to what? Being told I can't have a medical procedure I need?
These are all extreme examples, but they are the concerns I fear 10, 20, or 50 years from now. Small liberties, like being able to injest whatever the hell I want into my body should be my right. It's the abortion argument, right? It's *MY* body, let me do what I want with it.