Self-deceit is an inherent part of man's emotional, mental, and spiritual makeup. We have a hard time seeing it in ourselves because we are experts in hiding reality from ourselves and trying to hide it from others. However, we fool ourselves if we think we can hide our true nature from God.
The subject in which Jeremiah 17:9 appears is faith. God is pointing out why we allow our deceitful heart to get away with evading truth. It is a matter of faith—trust. Verse 6 shows that the person who does not live by faith will not grow. He will be like a shrub in the desert that only receives water every so often, not near often enough to grow.