For the word of God is living and active. Sharper than any double-edged sword, it penetrates even to dividing soul and spirit, joints and marrow; it judges the thoughts and attitudes of the heart.
God's word is not bland and soft, it is sharp and cuts us to the heart. It reveals things to us about the "thoughts and attitudes of our hearts". It shows us what we are really like and what God is really like. Reading the Bible should result in us being challenged and convicted of sin and confronted by the awesome, holy, loving, God. We should come away from the Bible with a greater awareness of our own sinfulness but also a greater assurance of God's love for us.
Read some of John Piper's thoughts on this passage here