I've the same question; basically, whether the "you" in the sentence is the implied subject or object of amare. Is this maybe indicated by which preposition is used ("per" here)? Or is that completely idiomatic?
For example, if the sentence were "The cake is too sweet to eat" would we say "La torta è troppo dolce per mangiare"? (In that case there is no ambiguity except maybe in a scifi flick.)
I think per is used when an action is intended where as da expresses obligation. Da is used in passive infinitive construstions. This link explains more clearly, but it's still a pretty tricky subject. http://www.italian-online.de/grammar/chapter13/13_4_4_connecting_with_da.htm