Izuku putting others' lives above his own can go to the extreme, to the point where he has little to no sense of self-preservation. Katsuki speculates that the bullying Izuku received for being Quirkless made him internalize that his life has less worth. Eventually, Izuku relented, adapting his battle style to put less strain on his body. Though when he's in a particularly tight situation, he'll pull back on old habits.