Leo Babauta on why people often fail at beginning new habits:
When you look at it this way, the key to forming a habit is not how much you do of the habit each day (exercise for 30 minutes, write 1,000 words, etc.), but whether you do it at all. So the key is just getting started.
Let me emphasize that: the key to forming a habit is starting each day.
While the idea seems obvious, exploring why we fail to start requires a deeper introspection. The specific reasons may vary, but Babauta describes their common origins.