You’ll read Tim Keller’s The Prodigal God in one sitting and here’s an excerpt from p38 to get you started:
Why is [the elder brother in Jesus’s parable in Lk 15.11-32] so angry with the father? He feels he has the right to tell the father how the robes, rings, and livestock of the family should be deployed. In the same way, religious people commonly live very moral lives, but their goal is to get leverage over God, to control him, to put him in a position where they think he owes them. Therefore, despite all their ethical fastidiousness and piety, they are actually rebelling against his authority. If, like the elder brother, you believe that God ought to bless you and help you because you have worked so hard to obey him and be a good person, then Jesus may be your helper, your example, even your inspiration, but he is not your Savior. You are serving as your own Savior.
What does God owe you that he’s not already given to you freely? Paul asked, “He who did not spare his own Son, but delivered him over for us all, how will he not also with him freely give us all things?” (Rom 8.32). I’m the elder brother who spends his time trying earn in futility what has already been given to me in Christ. We do not earn God’s favor. We enjoy it.