I'm my biggest danger. My own ego is my biggest danger. The counter part to that is when I go up, God goes down. So the Bible says, "Humble yourself under the mighty hand of God that in due time he may exalt you." We tend to turn it around and drift toward things that magnify us. And if you write books, or have a growing church, or whatever, the insidiousness of the temptation to feel good for the wrong reasons is almost invincible. You're on your face so often... at least I pray crazy prayers like, "If I can't emotionally distinguish between my delight in God and my delight in prospering, kill me. Take me out. I don't want to bring any reproach on the gospel.
John Piper, Q&A at Text and Context 2008