A. Because we're not:

JOH 8:7. So when they continued asking him, he lifted up himself, and said unto them, He that is without sin among you, let him first cast a stone at her.

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

B. We shouldn't think we are better than others:

ROM 14:2. For one believeth that he may eat all things: another, who is weak, eateth herbs.

ROM 14:3. Let not him that eateth despise him that eateth not; and let not him which eateth not judge him that eateth: for God hath received him.

1COR 13:4. Charity suffereth long, and is kind; charity envieth not; charity vaunteth not itself, is not puffed up,

PHI 2:3. Let nothing be done through strife or vainglory; but in lowliness of mind let each esteem other better than themselves.

C. A proud attitude displeases & angers God:

ISA 65:5. Which say, Stand by thyself, come not near to me; for I am holier than thou. These are a smoke in my nose, a fire that burneth all the day.

LUK 16:15. And he said unto them, Ye are they which justify yourselves before men; but God knoweth your hearts: for that which is highly esteemed among men is abomination in the sight of God.

D. It makes us despise & be unmerciful to others:

MAT 23:29. Woe unto you, scribes and Pharisees, hypocrites! because ye build the tombs of the prophets, and garnish the sepulchres of the righteous,

LUK 18:9. And he spake this parable unto certain which trusted in themselves that they were righteous, and despised others:

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.

MAT 9:13. But go ye and learn what that meaneth, I will have mercy, and not sacrifice: for I am not come to call the righteous, but sinners to repentance.

E. We only are what we are by God's grace:

1COR 4:7. For who maketh thee to differ from another? and what hast thou that thou didst not receive? now if thou didst receive it, why dost thou glory, as if thou hadst not received it?

1COR 15:10. But by the grace of God I am what I am: and his grace which was bestowed upon me was not in vain; but I laboured more abundantly than they all: yet not I, but the grace of God which was with me.