There are a lot of people who value history and its preservation greatly. 1000 years from now, people may only be able to read about and wish they were alive to see Notre Dame the same way we wish it about the Library at Alexandria today. Not to mention, it's an 800 year old bastion of Catholicism that survived a time when the foundations of that were being shaken into a thousand splinter religions. On an emotional level, that is tragic to me very close to on par with how I felt on 9/11.