Yes you can go out and shoot someone in the face. There will be consequences, but you can do it. It is both your and society's job to prevent you from hurting yourself and others. There are measures put into place which discourage this behaviour, and I advocate similar (though way less extreme) measures for regulating food content. By discouraging people from damaging themselves via their eating habits, you prevent a large amount of damage both regarding those individuals and for the society at large. You may not get the connection, but they are in the same category from a social engineering viewpoint. As such, it has a lot of bearing on the issue.
As for 'controlling ourselves', the government is us. We are a society that makes decentralized decisions as individuals and centralized, collective decisions via the government. People abdicate personal responsibility in favour of centralized decision-making, and that's fine so long as it works in people's best interest. I believe that encouraging healthy diets over unhealthy ones is in people's best interest.
I have less of an issue when someone does something that affects no one else in any way, but that's theoretical since everything people do affects others. Parents with bad diets train their kids. The obese cripple healthcare and skyrocket costs for everyone. Lack of physical fitness leads to lack of productivity and the ability to contribute your skills to the society at large over a long lifespan. Dying young can leave your family in the lurch. Excessive resource use could be better allocated elsewhere. When people get fat, it affects other people.