A third reason, God denies assurance to his most precious ones, is that they may be the more clearly and fully convinced of that exceeding sinfulness and bitterness that is in sin. (Jer. 2:19). Ah, Lord, says the soul that is sighing and   mourning under the want of assurance, I see now that sin is not only evil, but the greatest evil in the world, in that it keeps me from an assurance of my interest in thee, who are the greatest good in the world, and from an assurance of that favor of thine that is better than life, and from the light of they sweet countenance, that is better than corn, can alone make a paradise in my soul, (Ps. 4:7; 63. 3,4) Ah Lord! Now I find sin not only to be bitter, but to be the very quintessence of bitterness. Ah! No bitterness so bitter as sin, that keeps my soul from the sweet assurance, that is not only the  top and crown of mercy, but also the sweetener of all mercy, misery and glory. Oh what unspeakable evil do I see now in the evil that keeps me from the most desirable good! What bitterness do I find now in that which Satan, the world and my own deluded heart told me I should find sweetness in! Now I find by experience that to be true, which, which long since the faithful messengers of the Lord have told me, viz. that sin debaseth the soul of a man, that it defiles and pollutes the soul of a man, that it renders the soul most unlike to God, who is the best and greatest, and above all it renders  it most like to Satan, who is a very sea and sink of sin. That it hath robbed the soul of the  image of God, the holiness of God, the beauty of God, the glory of God, the righteousness of God, and that keeps the soul from wearing this golden chain of assurance.  [Thomas Brooks]