Purpose, as mentioned by #2, is the reason for continuing to live. Our purpose in life is laid out for us at birth (if we are born into money, or politics, or the military) or chosen during or after school (educational vocation, which classes did you really enjoy, what do you want to make money at) or sometimes it falls in our laps without warning (lottery, death in the family, realizing you are really good at "X" without studying or practicing). We make our own purpose in most things: we set a goal and strive to achieve it, we follow the line set out by our parents, we wander until something strikes our fancy.
Without a Purpose, we feel lost, adrift in a world that doesn't care if we live or die. We must in every case work towards something, even if that something is just living comfortably day-to-day. We cannot divine a magical or spiritual purpose to life unless we are spiritual in our beliefs and faiths; for many people, the loss of their faith removes their Purpose. For those who do not style their lives after religion, Purpose comes entirely from within, and is a powerful motivator. You, above all others, are the decision-maker when it comes to purpose.
To strive for the end, when we think the end is nothing and we take nothing with us and leave nothing behind, becomes depressing. To counter that depression, we create artificial purpose that may or may not be healthy -- booze, sex, money, power. We generalize our purpose based on our system of beliefs, whatever they may be. In the end, though, we leave others behind; their memories of us are what remains. If we leave a Legacy, it is concrete, and can be remembered by those we never met. If we strive to live for ourselves, and not allow others to hijack our living, we can leave behind something that no one can erase; good will and fond memories.