It pulls down the plumes of his pride!

(Thomas Watson, "The Ten Commandments")

The true Christian cannot keep God's law perfectly.
"There is certainly no righteous man on the earth
who does good and never sins." Eccl. 7:20. There
is in the best actions of a godly man—that which is
damnable—if God should weigh him in the balance
of justice. Alas! He cannot pray without wandering,
nor believe without doubting. "For the desire to do
what is good is with me, but there is no ability to
do it." Romans 7:18. Paul, though a saint of the first
magnitude, was better at desiring than at performing.

The regenerate have a desire to obey God perfectly;
but they lack strength; their obedience is weak and
sickly. The mark they are to shoot at, is perfection of
holiness. But though they take a right aim, and do
what they can—they come short of the mark!

A Christian, while serving God, is like the rower who
plies the oar, and rows hard—but is hindered, for a
gust of wind carries him back again! So says Paul, 
"For I do not do the good that I want to do, but I
practice the evil that I do not want to do." Romans
7:19. "I am driven back by temptation!"

God permits this inability in His people—to humble
us. Man is a self-exalting creature; and if he has but
anything of worth, he is ready to be puffed up! But
when he comes to see his deficiencies and failings,
and how far short he comes of that holiness and
perfection which God requires—it pulls down the
plumes of his pride, and lays them in the dust!
He weeps over his inability!
He blushes over his leprous spots!
He says with Job, "I abhor myself in dust and ashes!"

God allows this inability be upon us—that we may have
recourse to Christ—to obtain pardon for our defects, and
to sprinkle our best duties with His blood. When a man
sees that he owes perfect obedience to the law—but has
nothing to pay, it makes him flee to Christ, to answer for
him all the demands of the law, and set him free in the
court of Divine justice.