Long roads often have ditches on either side.
Sometimes we are encouraged by the things that we are taught to think of a confession of our inability as a sign of our humility; we are made to think for instance, that if we can perform anything that God requires, we would have a reason to boast. That isn't so. There is nothing to boast about in having only done what was our duty to do to begin with, using the ability that God gave us to begin with as well. Whatever the work is, whether it is believing, or doing, or not doing, willing, or not, there's nothing to boast about. God gave each of us our different capacities and abilities and He expects us to use them. There's nothing to boast about when we do.
The Bible says that all sin is unbelief. Confessing our inability may or may not be an indication of our humility. It may be a sign of our unbelief too. But we shouldn't think we have to constantly affirm our inability to do anything good, in order to avoid having a reason to boast. Doing good or not doing wrong may be a temptation for us to boast, but without any good reason for it.
It is said in the Bible that salvation is not of works, not because doing, or believing, or not doing, gives anyone a reason to boast - it is excluded because the only work worthy of boasting about is the glory of God, which we fell short of, which the Lord Jesus Christ accomplished(John 17:4-5).
On the other side, it may be that the religous leaders recieved some of the hardest rebukes from God, not only because they were confident of their own goodness and ability to perform what God requires, but because of their critical and harsh attitude towards others because of it(John 9:28-29,34). They were self-assured and highly confident and not afraid of condemning others in their judgments about them.
In Matthew chapter 7 verse 3, there are two questions really that are taken together as one: why do we behold the mote in our brothers eye, but not consider("observe fully" according to Strong's greek dictionary) the beam that is in our own.
As often as we look at the sins and faults of others, are we percieving our own as well?
Paul says the spiritual are to correct others with gentleness, humility, and considering themselves at the same time.
We are are best able to consider ourselves when we are looking at the perfection of God and Christ day by day. It is easiest to forget about our own sins and faults when we are looking at the sins and faults of others more and more every day.