פירוש על ברכות 48:5

Since it is regarded as an involantary act.

The verb "to sneeze" is also used euphemistically in the sense "to break wind,"

He was susceptible to sneezing and experienced it even when praying. Hence this teaching relieved his mind.

It is a sign his prayer will be heard in Heaven. Mr H. Loewe drew my attention to a Latin parallel : "Hoc ut dixit, Amor, sinistra ut ante, dextra sternuit approbationem" ; Catullus 45.