Because conflicts are usually the result of two incompatible things, where at least one has to be changed so the two things can then be in harmony. It's one of the most basic rules in life, and a well known fact for every story writer-- every story MUST include some kind of conflict, and usually a resolution of that conflict, to be interesting and enjoyable to people, because that's how life is too. For instance, a book might start with someone's normal life at school/home. Then the family moves, and the kid has to change to a horrible new school, gets in fights with the kids at new school, etc. The kid who moved could either hate it forever, which would be boring, kill everyone in the new school, which would be unrealistic (to most people), or realize that he/she was handling the situation wrong, learn and change.