Because humanism aims for human development. Humanism targets a better world here, now and in the future through protection of human rights and liberties, and participatory democracy.
Because humanism believes in human capacity to create a better world. It is the human capacity and responsibility to freely set up own life and find the truth through reason and science.
Becuase humanism rejects the superiority of human beings over other living species. Humans, being equal members of nature as other species, should live in harmony with nature and stand up against exploitation of all species.
Because humanism does not avoid any idea, embraces and tries to understand human nature with all aspects. Humanism rejects deference to supernatural beliefs, but not the beliefs themselves; feels responsibility towards values of society and humanity.