Have you ever started a novel where the main character decides to face an impossible task and an implacable enemy with the odds so far in favor of the bad guys that success, let alone survival, is minimal at best?
Sounds like a great novel, doesn't it?
Here are two novels I read where the main character deals with that impossible situation. In one novel, the hero must save his young daughter from a very ugly death. In the second, the heroine must find out the truth about the death of a young woman she's never met, and the outcome appears to have no real value to her. She's not even working for money.
I zipped through the first novel like a speed-reading lunatic to find out how the hero managed to save his little girl. The second novel I very nearly tossed away after the first few chapters because I hate stupid and suicidal main characters who have no real reason to go forward in an impossible situation, but I persevered out of curiosity and a fondness for dissecting author mistakes.
After over half the novel, the author of the second novel finally lets the reader know why the heroine has continued forward in the investigation, but by then, the damage has been done to the novel and the reader's reactions to the heroine. The reader also realizes that the author has cheated by withholding vital information which an author who is fair would not.
At this point, the odds of the reader picking up the next book by this author are slimmer than the original chance of the hero's survival.
As an author, you must balance goal, cost, and motivation. If the goal and the probable cost for the main character is great, the character must have motivation that equals both.