Because until you understand and believe in reincarnation, you will never understand life.
Reincarnation is simply the continuance of life.
Or that all life revolves in a circle. Einstein said that all energy follows a curve. If that is true, then all life follows that same curve. Every thought, every action, and every deed will go out from you, and return to you. Every thing you do in life, you will comeback and undo in the next.