If people drink too much, take drugs, smoke, engage in risky activities, or become enormously fat, goes the argument, then they shouldn't qualify for NHS treatment and they should be left to their own devices. If their behaviour leads to severe medical conditions, they should be left to seek treatment at their own expense and not expect the taxpayer to step in and sort them out.
Well, apart from the fact that the NHS has always pledged to be free at the point of use, regardless of personal circumstances and regardless of financial status, deciding that certain people shouldn't qualify for NHS treatment would be the start of a slippery slope. Once you begin turning away certain individuals, where would it end? The exemptions would proliferate until whole swathes of people could no longer use the NHS.
Furthermore, if people happen to have engrained psychological weaknesses that have led to illness - lack of self-control, recklessness, impulsiveness, whatever - why should be penalised for it? They may have tried many times to change their behaviour and failed. Or their behaviour might be a response to distressing personal circumstances they find it hard to cope with. People don't simply "make themselves ill".
Another case of engaging mouth without engaging brain.
NB: Of course all this only applies to the NHS because health care is organised quite differently in the States.