Why does Holden become upset when he sees someone has written "fuck you" on the wall at Phoebe's school?

Holden is waiting for Phoebe's lunch break at her school when he sees something written on the wall. He finds out it says "fuck you", he becomes upset and wipes it off. There could be many people who had written it, but Holden believes it was a phony. Someone trying to impress his friends or an older kid, trying to take away the young children's innocence. Holden is Phoebe's older brother, so of course he doesn't want her to see it, but he also wants to protect all the other kids from falling off the cliff, and losing their innocence. Holden becomes upset because no matter how many times he erases the "fuck you"s, ten other people will write it again.