Halliday & Matthiessen (2004: 631):
The potential for negotiation in dialogue created by metaphors of mood is directly related to the contextual variables of tenor. These are usually discussed in terms of status, formality and politeness. What they have in common is a very general sense of the social distance between the speaker and the addressee. Here interpersonal metaphor is part of a principle of interpersonal iconicity: metaphorical variants create a greater semiotic distance between meaning and wording, and this enacts a greater social distance between speaker and addressee.