Why should you try to score points in a video game? Why should your favorite sports team try to win the championship?
Or, to look at things from a less competitive and more altruistic perspective, read what philosophers have said about the matter -- namely, don't set out to accumulate karma; just set out to be a good person, and let your karma simply be a reminder of your legacy. Note: reddit makes no guarantees about attaining Nirvana.
Update: A redditor named jumpercable tried to redeem his karma. See how it went (your mileage may vary).