This is counter to all of our beliefs and desires. Here is a list of possible reasons but really why we give up on ourselves can only be answered by ourselves individually. Okay - there's a list of 20 odd potential reasons why anyone would give up. Those that are can be reasons. Those that aren't are excuses.
Excuses are inexcusable.
It is an honest question. Giving up happens when we no longer have the self-confidence to maintain persistence in achieving the task at hand. Giving up doesn't happen because you're bored. If it does then you must doubt your reasoning for its commencement in the first place. Your tasks and goals need to be as much within your control as possible.