A. Because we are not:

JOB 15:14. What is man, that he should be clean? and he which is born of a woman, that he should be righteous?

JOB 15:15. Behold, he putteth no trust in his saints; yea, the heavens are not clean in his sight.

JOB 15:16. How much more abominable and filthy is man, which drinketh iniquity like water?

PSA 130:3. If thou, LORD, shouldest mark iniquities, O Lord, who shall stand?

PRO 20:9. Who can say, I have made my heart clean, I am pure from my sin?

ECC 7:20. For there is not a just man upon earth, that doeth good, and sinneth not.

ISA 64:6. But we are all as an unclean thing, and all our righteousnesses are as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.

ROM 3:10. As it is written, There is none righteous, no, not one:

ROM 3:23. For all have sinned, and come short of the glory of God;

ROM 7:18. For I know that in me (that is, in my flesh,) dwelleth no good thing: for to will is present with me; but how to perform that which is good I find not.

B. Only God is good:

MAT 19:17. And he said unto him, Why callest thou me good? there is none good but one, that is, God: but if thou wilt enter into life, keep the commandments.

JOB 9:2. I know it is so of a truth: but how should man be just with God?

PSA 71:16. I will go in the strength of the Lord GOD: I will make mention of thy righteousness, even of thine only.

C. Doing so causes us to be lifted up in pride:

PRO 30:12. There is a generation that are pure in their own eyes, and yet is not washed from their filthiness.

PRO 30:13. There is a generation, O how lofty are their eyes! and their eyelids are lifted up.

LUK 18:11. The Pharisee stood and prayed thus with himself, God, I thank thee, that I am not as other men are, extortioners, unjust, adulterers, or even as this publican.

D. It separates us from God:

ROM 10:3. For they being ignorant of God's righteousness, and going about to establish their own righteousness, have not submitted themselves unto the righteousness of God.

JOB 13:16. He also shall be my salvation: for an hypocrite shall not come before him.

E. It makes us unable to see our sins:

JOB 32:1. So these three men ceased to answer Job, because he was righteous in his own eyes.

PRO 16:2. All the ways of a man are clean in his own eyes; but the LORD weigheth the spirits.

PRO 21:2. Every way of a man is right in his own eyes: but the LORD pondereth the hearts.

PRO 30:12. There is a generation that are pure in their own eyes, and yet is not washed from their filthiness.

LUK 6:41. And why beholdest thou the mote that is in thy brother's eye, but perceivest not the beam that is in thine own eye?

LUK 6:42. Either how canst thou say to thy brother, Brother, let me pull out the mote that is in thine eye, when thou thyself beholdest not the beam that is in thine own eye? Thou hypocrite, cast out first the beam out of thine own eye, and then shalt thou see clearly to pull out the mote that is in thy brother's eye.

JOH 9:41. Jesus said unto them, If ye were blind, ye should have no sin: but now ye say, We see; therefore your sin remaineth.

F. True righteousness means trusting God:

GAL 3:6. Even as Abraham believed God, and it was accounted to him for righteousness.

PHI 3:9. And be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith: