When lovers look into each others eyes, they are looking to see instinctively, (relax

ladies and gentlemen ) if the others' eyes have large pupils.
It is instinctive for the eyes to attain that quality, in that.. ahem...state. Something to do with hormones

and oxygen
So there is some science behind it, not just spirituality and romance.
Of course, the last two are probably the most applicable

and who cares about science, anyway (at least in that situation)