Halliday & Matthiessen (2014: 178):
Proposals that are clearly positive or negative, as we have seen, are goods-&-services exchanges between speaker and hearer, in which the speaker is either (i) offering to do something, e.g. shall I go home?, (ii) requesting the listener to do something, e.g. go home!, or (iii) suggesting that they both do something, e.g. let’s go home! They rarely have third person Subjects, except as prayers or oaths.