When I think about the dynamics of sin, I find it helpful to look at Genesis 3:1-6, which is probably the finest short exposition of how sin works there is. The passage reads as follows:
3:1 Now the serpent was more crafty than any other beast of the field that the Lord God had made. He said to the woman, “Did God actually say, ‘You shall not eat of any tree in the garden’?” 2 And the woman said to the serpent, “We may eat of the fruit of the trees in the garden, 3 but God said, ‘You shall not eat of the fruit of the tree that is in the midst of the garden, neither shall you touch it, lest you die.’” 4 But the serpent said to the woman, “You will not surely die. 5 For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil.” 6 So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate (ESV).There's a lot that can be said about this passage, but for now it's good just to notice that the serpent made God out to be a liar who was depriving Eve of what was rightfully hers and would benefit her greatly if she had it. Once Eve believed this about God the outcome was obvious; she would take the fruit, just as I would have had I been in her place.
In fact this is how I justify much of my sin today; by telling myself that God is wrong to have forbidden what he did, that I'm entitled to have it and that it will greatly benefit me (or others, if I flatter myself into believing that I'm altruistic). As a result, I grab what I want, despite God's prohibition. Or, if I am too weak or afraid to impose my will on the situation in which I find myself, I don't grab what I want, but I harbor in my heart a deep resentment against God for placing me in a situation which prevents me from having what I need and deserve.
What happens when I become a Christian? Hopefully I am less prone to claim that God is wrong when he forbids things that I want and more likely to believe that some (at least) of my desires for things may be wrong. But I still encounter things that God appears to be forbidding (or at least refusing to make possible) that are (to my eyes at least) manifestly good or even necessary. What do I do with these?
In C. S. Lewis's allegory The Pilgrim's Regress the main character John asks the heroic lady Reason what would happen "if a man wanted to know [something] so badly that he would die unless the question was decided --- and no more evidence turned up." Reason's answer was simple; "[t]hen he would die, that would be all." Is it possible that if I were to ask God what would happen if I encountered something that I urgently needed and could not get, that he would also answer me "then you would die, that would be all?" Is fatalism the answer to possible disappointment?
I don't think that God wants us to stop caring or to stop striving for what is good or to stop petitioning him to give us what we and others manifestly need. But I need to remember that if God chooses not to give what is good, it is not because he is indifferent to my situation. The worst that can happen to me in this life if God fails to answer my prayer is that I die - and then I go to heaven. I may not get the job I want or my family may not treat me well or society may make terrible choices, but in the end we all die, and what happens then? Do I really believe that God has done in Christ all that needs to be done to make good the suffering and evil in my life and in the lives of those I love, even this present suffering that I find so intolerable?
If I don't really believe this, then I will find myself furious at God for allowing this evil, convinced that nothing can justify the suffering I now endure. But if I look at the cross, I sometimes realize that maybe there are reasons why God permits unjust suffering, and that if I or someone I love finds ourselves in that position we are in good company. And that realization may help the inner urges to subside, if I realize that if they're not satisfied "I'll just die," and that that's not such a bad thing.