I find it comforting to now that only God has access to our thoughts. Sometimes it seems that Satan knows exactly what I'm thinking when he throws his temptations at me. At those times I remember this scripture and remind myself that Satan is only guessing. He knows me, because of my relationship to him in the preexistence, so he has an idea of what I might be feeling or thinking, and that's why so many times he hits the mark with his guesses.
We probably don't really notice the times when he guesses wrong because the thought or impression simply slides away unnoticed since it doesn't relate to what we are thinking. We only notice his efforts when he guesses right--that's why it seems that he knows our thoughts.
The Lord does know our thoughts, and this allows Him to truly know us. It helps me to remember this when Satan attacks me, giving me the strength to turn to the Lord and seek his guidance and protection.