No one can have peace within themselves when the guilt of sin lies within their heart, causing a broken relationship with God. Psychology has realized for many years that guilt causes insatiably, irritability, and anxiety. What psychology has found out, however, the Bible has taught all along. He is speaking about those who are weary and weight down with the burden of the guilt of sin. Until one gets rid of the guilt of sin they will never have peace of mind.
Fear also robs people of peace of mind.
Yet, I suppose the greatest fear that is shared by all is the fear of death. We must simply be ready to die, for unless we are ready we live in fear. In Christ one needs not to fear death. Through Christ there is no fear of death because there is hope of eternal live, but without Him there is only a fiery expectation of judgment to come. Worry, too, is something that keeps many from having peace of mind.