Why do people say 'must' when they mean no more than 'should' (as in 'The government must act to etc', 'The England selectors must drop Ian Bell', 'You must write to thank her')? 'Must' carries more force than 'should' through sometimes connoting genuine compulsion. The compulsion can be of different kinds. 'These policies must fail' is a prediction that, the world being as it is, these are policies that cannot be successful, so that the 'must' expresses an inevitability. Or 'must' can imply that the speaker is in a position vis-à-vis the spoken to enabling the former to demand something from the latter and compel him if he declines to comply voluntarily. Thus, 'You must vacate the property or else face legal action'.
So, to rephrase my original question: why do people say 'must' when the circumstances of the utterance clearly indicate that the 'must' has no compulsion behind it, neither the compulsion of some (putative) causal inevitability nor the compulsion of assumed or real authority? Perhaps just to add rhetorical force to what they say. But if that's all, then 'should' will serve equally well. No one is going to think, 'Cripes, she said "must", so we must.' Or perhaps 'must' is supposed to carry an implicit 'or else'. But there's always an implicit 'or else', even with moral judgements expressed only by 'should' or 'ought to'. Therefore I say: say 'should' in preference to 'must' unless your 'must' really means business.
But I must get on.