I've encountered this in songs. If I remember correctly, both times it was "恋しようよ" so i assume this is a form of "(恋を)する"? Also is the よ the particle you add for certainty?
~よう is a verb suffix that expresses volition, proposition, or invitation, akin to "let's" or "shall we?" in English. [Source]
する → しよう. The よ makes it slightly persuasive, a bit pushy (depending on tone). I guess "emphasis" is the closest you'd get in a dictionary sense.
PS: 恋しよう more or less means "Let's fall in love".
Edit: I guess it's a bit more like "Let's be in love", I'm pretty sure it's not strictly about the moment you "fall in love". It's kinda framing love as an activity/state you do together. Does that make sense? It might not be worth overcomplicating though.
