As Michael Horton has pointed out, humans identify with law and works. It is our default position. Faith and trust in Christ alone, i.e. dependence on what Christ has done through the cross, is counter-intuitive and itself a gift of grace. Paul in Rom. 3:21, after showing how all are shut up in sin under the law, introduces the good news of justification by faith with the exclamation "But now!". We hear that good news and too often instinctively ask,"Yeah, but what do I need to do?" It's as if God has only started our justification through faith in Christ and now, in order to complete it and maintain it, we need to make sure we're holding up our end of the bargain.
Indeed, there are good works. But they necessarily proceed from our justification in Christ. Trusting in Christ alone for one's salvation results in a new heart (a work of the Holy Spirit) that is now alive to God and inclined toward obedience. Thus works flow from faith and thankfulness in light of his wondrous gift of forgiveness and justification. But these works (being yet imperfect) are never, partially or whole, the basis of our right standing before God. That righteous standing is secured solely on the basis of Christ's finished work of redemption. So to say "by faith alone" is to say "by faith in Christ's finished work of the cross alone", i.e. being declared righteous before God because of his perfect sacrifice for our sins and his perfect obedience on our behalf for our justification. Our motive then for obedience is one of thankful duty, not one of need or fear.