Most people are familiar with "The Parable of the Prodigal Son." We don't often realize though that Jesus didn't name the parable. Really, we should call this the parable of the prodigal sons or the parable of the loving father. Either way, I think we all relate to either one son or the other.
The first son lived in a reckless and licentious way. He was wild and squandered his inheritance. His asking his father for his share of the inheritance was essentially saying, "I wish you were dead. I'm not interested in you, but in your money." He presumed upon and then trampled the grace and love of his dad.
The second son was a rule keeper. According to tradition, he would get the majority of the inheritance when his father died. He was following his dad's rules and would appear to be honoring him. We later find out though, that when his brother returns home, he refuses to go into the party being thrown. He's angry that his brother is back. In fact, he refuses to even call him his brother. He's jealous that his father never even offered so much as a young goat for he and his friends to have a party with. In his mind, he's earned the inheritance coming to him and he was just waiting for his dad to die so he could have it.
When the younger brother returns home, the father throws a party. He's overjoyed that his son has come to his senses and has come back home. He graciously accepts him back and not only that, he throws a huge party in celebration. In his repentance, the younger son joins the party!
The older brother refuses to come in so the father graciously comes out and invites the older brother into the party (he shouldn't have had to do this and it would have been ok if he hadn't). The brother is resentful. He says, "I've kept your rules. I've done everything right. Why would you throw a party for this other son of yours?" We aren't told if the older brother ever goes with his father into the party.
The first son abused the love and grace of his father. The second son didn't want his father's love and grace; he wanted to keep the rules. We know in this parable that the father represents God. In Jesus time, the younger son represented the tax collectors, prostitutes, and people who sinned in very public ways. The older son represented the Pharisees and teachers of the law. Both were outside God's kingdom (the party), but in two very different ways.
Jesus was communicating that God invites both types into the party. He invites those who abuse grace and those who try and follow the rules and earn grace. He invites them both in solely based on what Jesus has done for sinners. You can't earn it or be good enough, but at the same time you haven't been too bad to be given it.
I think we all typically fall in one of these categories. We either have the tendency to think because of Jesus, I can do what I want and He has to forgive me. Or we can think that God is lucky to have us because we obey Him so well. Both are wrong. Our salvation is based solely on the finished work of Jesus in our place. Which brother do you relate to?