What would I say?
"She's the most beautiful person I have ever met in my entire life".
"She makes me the happiest person in the whole wide world".
I'd say other things too, depending on who asked, and depending on what time of the day it was, and depending on where I was. But I'd say things if some fella asked me, even if I were in a hurry, even if I didn't know who that person was, even if it was me who was asking the question myself.
But I wouldn't know exactly what I'd say, really. You never know these things so surely, you know? Depending on the time of day, where I am, and how well I knew that fella who was asking.
But you know what, love?
It is gonna be a very, very good thing if fellas keep asking me, "why are you in love with her?"