Halliday & Matthiessen (2014: 194):
For any clause, there is one choice of Subject that is ‘unmarked’ — that is assumed, in the absence of evidence to the contrary. In a giving clause (offer or statement), the unmarked Subject is ‘I’; while in a demanding clause (question or command), the unmarked Subject is ‘you’. This means that, if a clause that on other grounds can be interpreted as offer or statement is without a Subject, the listener will understand the Subject [as] ‘I’ — that is, Subject equals speaker … Whereas if it is a question or command the listener will understand the Subject [as] ‘you’ — that is, Subject equals listener …