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Reasons for Procrastination:

Difficult - the task seems hard to do; we naturally tend to avoid difficult things in favor of those which seem easy to us.
Time-consuming - the task will take large blocks of time, and large blocks of time are unavailable until the weekend.
Lack of knowledge or skills - no one wants to make mistakes, so wait until you learn how before you start.
Fears - everyone will know how you screwed up.

-From the above article.
Highly recommended read for anyone with a tendency to procrastinate.