Natsume’s greatest fear has come true.  Someone not involved in the world of youkai is hurt because of him. Natori needs Natsume to regain his composure or they will be eaten by Omibashira!
And Sensei was saying it's because
El profesor me dijo que por 
entrar al mundo de los youkai,
El maestro me dijo que al entrar en el mundo de los youkai,
E, segundo o Mestre, como estou no mundo
 dos youkai, consigo vê-los como você os vê.