As a student, I was "good at" math. When I took Math for Elementary Teachers during my undergraduate degree at Simon Fraser University, I realized that I didn't know WHY certain formulae exist in math. I knew what they were, but not why they existed.
With my own students, I have enjoyed seeing them discover how to use Pi to calculate the circumference of a circle:
Many students were intimidated by "pi" before learning what it is and how it works. Now, they are proud that they can use and understand it. Breaking this activity down so that they could really figure it out helped them to feel more confident about a concept they previously thought was difficult.
As well, I have enjoyed seeing them realize WHY base x height / 2 = the area of a triangle
I love incorporating real-life and hands-on examples into my math classes. As we do things like this, greater understandings of concepts follow.