Thought -- or rather, more specifically, taking thought seriously, 'identifying with it' -- is the root of all our dissatisfaction, unhappiness, and existential confusion. To pierce the identification, we repeatedly attempt to discern the background against which thoughts occur, and the end result is that identification is realized to be a fiction. No one was affected by it, no one pierced it. But paradoxically, great effort is required for this to happen.
The mysterious nature of identification with the ego
What does it mean to identify with the ego and thus with the body and mind? It's a very elusive thing.