Imagine how you would feel if someone kept trying to earn a gift you had given them for free. As an expression of your love for them you gave them a gift that was all they wanted and more, yet as time went on they kept trying to do good things for you or pay you for the gift. They in essence were failing to recognize the love you were showing and the extent to which you showed it by giving them a free gift.
Or imagine that you have an insecure spouse who has a history of being abandoned by parents or loved ones. Your commitment to them is free, unconditional and bound by covenant. However, you find that they keep trying to please you so that you won't leave them. Would you feel hurt that they weren't trusting you to love them freely? Would you find it annoying that they couldn't trust you to offer your devotion to them without requirements on their performance? In high school I dated someone like that. She had been abandoned or abused by men in her past to the point that she was not prepared to accept that a man might be loyal to her simply because he had determined to be loyal. She tried everything to make sure I wouldn't break up with her, even though I wasn't going to anyway. The relationship eventually fell apart because she could not bring herself to accept loyal love and unconditional acceptance from a man.
Many of us are like this all the time. For everything else in life we work and strive to earn something. We have a tough time imagining God offering his loyal love as a free gift because nothing else is free. The saying goes "There ain't no free lunch." We find this to be true in much of life so we think it applies to God. The gospel winds up being a terrible conundrum for us because in it God is operating opposite from how we would expect. We would expect all sorts of conditions to be given up front: "clean up your act, watch your language, quit smoking, drinking (or any other drug for that matter) and then we'll talk about how deserving you might be of salvation from judgment in Hell." People think like this all the time. You hear it when they say, "I think I'll go to Heaven someday because I'm a good person." That's conditional salvation and is completely opposite from the free gift of grace that God has offer through the completed work of Jesus Christ.
I'm even susceptible to this tendency. At times when I've been a "good boy," I think God is closer to me. At times when I've been bad, I think he's distant. When I think that, I'm basically making the efficacy of Christ's work on the cross in bridging the gap between God and me conditional upon my behavior. This is not the essence of God's grace. Instead it is by grace alone that my relationship with God is both offered and maintained. Sola gratia was a pillar of the Protestant Reformation because the roman church had slowly began to teach that it is by our cooperation with God in taking the sacraments and holy living that we earn merit applied to us from the "treasure chest" of righteousness stored in heaven. Rome even went so far as offer these payments of "merit" applied to one's account in heaven in exchange for supporting the church financially. Indulgences were sold as a means of supporting the church's building program and (it was taught) a means of applying merit to one's account. This "merit" (or credit earned by the good deeds of Christ or other saints) could counteract one's bad deeds in the final accounting. If someone bought an indulgence, it could be applied to your heavenly account or a loved one's account now languishing in purgatory being cleansed of their impurities before entering heaven. Salvation was not a free gift then; it was for sale at closeout prices.
Sola gratia must return as a battle cry of future reformers. We so easily and quickly forget it. Consider how tempting it is for us motivate people to give for our building program by suggesting God will be more pleased with them if they give. Heresy! A portable building used for an exciting new children's ministry would be a great blessing, but IT SHALL NOT BECOME SAINT PETER'S BASILICA. People should give because it is good that people give. The Scriptures teach that the people of God are to be a generous people. But people are NEVER to be taught that the pleasure of God is for sale.
Sola gratia also reminds us that behavior follows belief, not vice a versa. While the discussion in some circles continues as to how much behavioral change should follow initial belief and how quickly it should be evident, the order of belief preceding behavioral change is NOT open for discussion. God graciously initiates the relationship with us through the illuminating work of the Spirit and then goes to work on us as disciples of Jesus Christ. God's loyal love is offered freely by grace alone. There's nothing we can do to make him more gracious to us. This remains a comfort that my relationship with him now, and my place in heaven someday is as secure as his gracious character. There is no greater comfort.