This is a such a big change from Act One All Might. His first reaction to Izuku experiencing haunted One for All during the Sports Festival was to shut down anything he didn't understand or experience and call it scary.
By the Joint Training Arc, he was actively listening to Izuku's concerns and considering the opinions of others on One for All. He was willing to acknowledge the problem, admit that he isn't all-knowing, and reassure Izuku that they would figure out this mystery together. This scene demonstrated significant character growth for All Might in his role as a teacher.














