It changes because in the beginning he is oblivious to anything he's done wrong. He doesn't feel like he's contributed to Eva smiths death. As the play goes on he learns that in fact he might have had an impact on her death and he takes more responsibility for it so the level of guiltiness gradually gets higher as he hears more about her death. At the end he feels extremely guilty and decides to change instead of ignoring what's happened like Mr and Mrs Birling
