It breaks my heart every time i visit Badshahi Mosque, Lahore Fort, Jahangir’s Tomb etc. There’s always a huge number of ‘writings on the walls’. Messages, declaration of love, phone numbers and so forth.
I’m sure that you guys do not do it but make sure that if you have kids with you, tell them that it is a stupid (to me, a criminal) thing to do, specially as tourists in foreign countries. We all have a responsibility to preserve and protect world heritage, be it the Badshahi Mosque, Eiffel Tower, Taj Mahal or the Great Wall of China.
Here’s an example of Vandalism at Great Wall of China