Halliday & Matthiessen (2014: 301):
The usual unmarked present tense for behavioural processes is present in present, like the material (e.g. you’re dreaming!); however, we also find a simple present in its unmarked sense (i.e. not meaning habitual), e.g. why do you laugh?, alongside why are you laughing? (with scarcely any difference between them), which suggests an affiliation with a mental.