I would assume because "di' is normally used for places while "a" seems to used for people.....this is just a guess though
I noticed that i can memorize in Italian only such kind of romantic sentence...
Nice! Always love to see comments mentioning Bocelli :) He is one of the reasons why I'm learning Italian right now haha.
Can it be used in a polite form and as result of it the translation to English be: I think of you day and night?.
I principle, yes, but in this case the pronoun would/should be capitalized: Penso a Lei giorno e notte.
I would have said so, but I you generally only use formal for those people you don't properly know, or to talk to professionals/officials, or for your elders, so that would be a bit creepy.
How am I supposed to know 'a lei' means 'of her' when 'of' isn't even listed when the cursor hovers over 'a'?