There's often a sense at the beginning of a clutter bust that, "I'm not going to find much to let go of." That makes sense. A person is used to living with their things. Generally these things seem to have a place in the person's life.
But upon careful consideration of their things, by singling out items and asking, "Do I love and use this, or can I let it go?" it becomes apparent and surprising to people how many of their things no longer serve them.
Once that realization kicks in, the clutter bust picks up speed. The person feels less protective of their things. It's replaced by an expectation that they will definitely find things that have outlived being useful in their lives.
This was summed up by my client a few days ago who said, "I can't believe how many, 'No, I don't need it's' I found!"