why do you think katniss is the way she is at the beginning of the novel?

In Chapter One, we quickly learn that there are reasons Katniss is the tough, determined, take charge, and aloof character we meet early on. She has chosen to adopt an "indifferent mask" so as to avoid becoming the woman her mother became after her husband's death – an emotionally overcome person who was incapable of providing for her family. In fact, it seems that she has eschewed passion and tenderness ever since her father's demise. Katniss forces herself not to consider any romantic feelings for Gale, though the reader sees right away that this is somewhat disingenuous. She also has no playfulness in her life. As strong as she is, we should never forget that she is 16 years old. Instead of allowing herself the joys of childhood, she has transferred all hopes for childish innocence to her sister Prim, who serves as a personification of innocence for her. She not only refuses to allow Prim to hunt, but is also convinced she will spare her sister whatever hardships are possible. So while Prim gets to stay an innocent, Katniss has become an adult, trading in the Hob, acting as provider, and throwing away childish things. All of these elements are set up in Chapter 1 to be challenged throughout her adventure.