Halliday & Matthiessen (2014: 217, 218):
The world of our experience is highly indeterminate; and this is precisely how the grammar construes it in the system of process type. Thus, one and the same text may offer alternative models of what would appear to be the same domain of experience, construing for example the domain of emotion both as a process in a ‘mental’ clause … and as a participant in a ‘relational’ one …
Emotion is one of a number of experiential domains that are construed in more than one way by the grammar of transitivity. Such domains are experientially difficult to come to terms with, and the grammar solves the problem by offering complementary models for construing them.